A top-tier character is the best character in the game,generally speaking.What determines them will probably be your next question;in MK:D's case,it's the character with the best and savest 50/50 mix ups in the game.The character's throws as well as his/her moves,or any other over-powered strategy,do matter as well.
From my playing experience,I'd rate the characters like this in MK:D...
1.Bo Rai Cho
2.Darrius
3.Scorpion
4.Dariou
5.Jade
6.Noob-Smoke/Tanya
7.Etc.
This is NOT an official list or something as many,many players tend to disagree over top-tiers.In any case,this game is completely broken and barely playable without any frustration.It's very simple to play due to the 50/50 mix ups.

Yea there really is no best character. No character is better than the other. It's just your personal preference and who you like. Anyway, some of the most powerful characters in the game are females i.e. Jade, Tanya, Ashrah. They're pretty good (but then again that's just me). So it's all comes down to who you like and who you can play with well.

i think Mileena was the best....she went all out and she reached a lot of people's expectations....i think she was the #1 female in the game......she was flawless

the top tier in this game due to their cheese and broken stuff r...
1~bo rai cho~broken
2~darrius~broken
3~tanya~broken
4~dairou
5~noob~smoke-because of smokes infinate
6~kobra

I appreciate the detailed responses. I'm trying to familiarize myself with the terminology used on this board so my next question would be what does "broken" mean exactly? Also, what is meant by "50/50"? Thanks again for the responses thus far.

Broken = over powered / cheap.
I'm not too sure about the 50/50 though.
I think it's about the high low combinations and risk and reward.
A strong attack should have higher risk a weak attack should have low risk. A pop up should have high risk...........
I'm not 100% on that, but that's the general idea I have.

"Broken" is a term that means there is a lack of balance or that characters do not play well together. One will have a far better advantage over another. "Broken" can be applied to either a single character in a game or, if the game has many broken characters, to an entire game itself. MK:D falls into the latter category since characters like Bo' Rai Cho are so bizzarely good while characters like Kabal suck so bad. In Tekken 4, Jin is often referred to as broken and in Marvel Vs. Capcom 2, Cable, Sentinel, Magneto, and Storm are considered broken. It's not possible to beat a halfway decent Magneto with Roll. Magneto has too much of an advantage. Therefore, MVC2 is broken.
50/50 is easy to understand. Simply put, 50/50 means that in a given situation there is more than one option open to the player, usually offensive in nature, that are each equally capable of being used. For example, if I am attacking someone and I have two moves, one a high and one a low that are both comparably fast and strong, then it’s a guessing game for my opponent as to which one to watch out for. If I use the low attack and he blocks high then I get the hit off not because I have more skill but only because he guessed wrong. 50/50s are good to have in fighting games but not so much so that the entire game revolves around it. Otherwise two players who are both very good at the game end up playing rock, paper, scissors. There’s no skill in rock, paper, scissors. The loser only loses because he guessed wrong.
For those who still don’t understand what 50/50 means I’ll use baseball as an analogy. Let's say two guys are playing catch. Mark throws a ball overhand at David’s head ten times and David catches it all ten times. To catch that ball David has to have the palm of his glove facing straight ahead. Think of Mark’s throwing the ball as an attack and David’s catching the ball as a block. So David just blocked all ten of Mark’s high attacks.
Now, let's say on the eleventh throw Mark decides to throw the ball underhand so David has to catch it differently with his palm facing up instead of straight. David catches that one too. So David just successfully blocked a low attack with a low block. Why? Because he can see what Mark’s arm motion is and where the ball is going. Mark’s body language told David that this next throw was going downward so he knew that he had to change his catching style to accommodate.
Now let's assume the laws of reality are broken and Mark can throw the ball overhand and underhand at the exact same speed. Very, very fast. Faster than David can see it coming. So fast that by the time Mark is done with the throwing motion David already missed the ball. How does David catch it? Does he put his glove facing out to catch an overhand throw or does he put it facing up to catch an underhand throw? There's no way for him to predict how Mark will throw the ball so if he misses then it's only because he guessed wrong. It's completely random hence 50/50. There's a 50% chance Mark will throw it one way and a 50% chance he’ll throw it the other. Because there's no way of seeing or predicting how Mark is throwing then it's nothing but random guessing on David’s part.
MK falls into this all the time because it’s so stiff and choppy. Players can’t see what’s coming next so the characters who have the best 50/50 options end up being broken. Often times the loser of a match will lose only because he guesses wrong more times than his opponent. It should take skill to force an opponent into a 50/50 guessing game, not be right there for someone to abuse. To force an opponent into a 50/50 situation should take some skill and not be a given. MK is broken because characters like Dairou can force someone into a 50/50 situation with no effort whatsoever.

How would you fix the 50/50 situation?
Make low attacks slower?
This is confusing, because there are very fast low attacks and very fast high attacks that can be done in real life.

Most of the time low attacks in 3d fighters are higher risk than highs and mids. But that isn't really the main concern. In MK the animation is so jerky that you can't see anything coming. The attack speed can actually stay relatively similar but if the animation was better players could have a better sense of what's actually happening. Of course, this would also mean the damage done by low attacks would need to be fixed. Low attacks doing 4% damage doesn't make sense. I know it doesn't make sense when compared to reality though. I can't really think of any way 50/50's can be toned down though based on a real life template. Probably because in real life nothing is precise. In a game a 5 frame poke is a 5 frame poke every single time. That needs to be taken into account since in real life speeds and stuff vary tremendously. There's no rule in real life that a strike has to beat a throw. Games need to be based around something exact even if it's not accurate to a real life situation.

My opinion is exactly the same as mentionaed above. From my experien-ce, the following fighters have prooved to be killing machines in my hands:
1. Bo Rai Cho
2. Sub-Zero-in dragon his main combos are unbelivably hard to avoid.
3. Hotaru-since he got now pi gua, he is a very powerful fighter+naginata
4. Scorpion-same as Sub, but much of his combos can be forgotten.
5. Shujinko -varies with opponent
6. Tanya/Jade-i like them almost the most.
Fighters that are weak or have not yet unlocked their full potential:
1. Mileena-extremely fast, but they drasticaly need to increase her damage
2. Darrius-practicing this one now.
3. Baraka-except for blades, he can be forgoten, easy to fight with him, but also easy to block him.

I am not a good deception player, but if I was, I would use JADE. I think she is the best overall character, but you have to be ungodly good cause she is hard to play....I play nightwolf, kabal, and baraka. mainly cause they are classic characters I have grown to like.

How would you fix the 50/50 situation?
Make low attacks slower?
This is confusing, because there are very fast low attacks and very fast high attacks that can be done in real life.
Animation is one crucial issue that has been already mentioned by Tony,but what's also important is a decent wake up game.In MK:DA and MK:D,when you are hit,and ultimately grounded,you automatically get up because that's how the engine works.And thus,it is this that causes those annoying 50/50 mix ups.A simple wake up game can be implemented so that a character can remain on the ground,while simultaneously having rising kicks as well.Play Tekken and see how it works.Of course this would call for moves that hit grounding opponents,and hence,there would be some kind of Oki.
It is best,however,that you go online and play some good players to fully understand the concept of the 50/50 mix ups.

I posted this on www.gamefaqs.com earlier,so I thought why not here as well?
Bo Rai Cho - broken because of his weapon stance;after f+3 (which hits mid and a has a huge hit box) a throw is guaranteed,and after his throw you can dash in and do b+1, u+2,4,4,u+3.All guaranteed if you don't use your combo breaker;about 65% of damage.Yes,Boon is a moron.
Dairou - great keep away due to his UNblockable tombstone stomp (which hits full screen and cannot be avoided at all unless you jump);after this,he does have some decent combos,especially with the juggle starter that turns him around most of the time (second stance,u+2);he also has a projectile to annoy you with and some kind of teleport.The bottom line is,you'll be jumping a lot and eating lots of projectiles.What a dumb character...^_^
Darrius - weapon stance ('nuff said);his 3 in his weapon stance hits mid and juggles;upon juggling,do QCF+3 and the opponent will not be able to tech roll after it giving you 50/50 mix ups;oohh and by the way,after his throw he can do about 60% of damage ANY TIME...:sigh:

Mileena was not the first number one character in MK...Kitana was (and still is). Kitana is way better and a lot of people like her way better than Mileena because she's just better than her. Mileena is just a stupid clone. (Not even Kitana's real sister)

Do you really like Dairou?
To me he is horrible, complicated combos and worst moves.
This is to me.
My top tier is Sindel. I like her combos and styles.

trust if it went by who i like in mk deception sindel would be # 1. the fact is if there werent any glitch combos w/ bo rai, tanya and darrius. dairou would be #1 followed by kobra.

Tiers aren't based on personal preference. They're based on numbers, logistics, and other difinitive factors. It doesn't mean the best guys in the game will be the ones you play the best with or like the best but in the end, Dairou has an easier time beating Sindel than the other way around.

haha but I do agree