I usually pick Sub-Zero (one of my all time favs). I also tend to use Baraka sometimes. Other than that, I basically use whoever I feel like.

i mainly only play as characters i like (which is amazing that... for the first timer... this is the first mortal kombat game in the history of mk where i liked more characters then i hated! (O_O)!!!), sometimes i just go onto random select... if it picks someone like raiden, tanya, mileena or shujinko, i'm on the floor feeling that i am going to puke... ok...
but my utmost best character is ashrah, she is the fucking power house in my opinion, chou jaio and ba gua are her best, kriss is good but not as good as chou jaio and ba gua.
side characters i am good at are ermac and kabal.

Kabal, Nightwolf, Darruis, Subzero and Li Mei Those are the ones I use the most :)

Mileena mostly. I also use Ashrah, Ermac, Tanya, and Kenshi (sometimes). I do use all of them these I just happen to choose more than the others.

I vary.
Sub-Zero is my primary fighter
Scorpion is my second mos used fighter
Baraka third
Jade 4th
Darrius 5th
Kabal 6th
Mileena 7th
sometimes I switch it up and play characters I don't ever normally play.

edit

I play with:
- Jade (extremely powerful).
- Raiden (powerful too).

My BEST characters in the game that get me easy wins are:
Mileena
Li Mei
Sindel
Other great characters that I use a lot:
Kabal
Nightwolf
Liu Kang
Kobra
Kira
Rayden
The ones I RARELY (almost never) use:
Scorpion
Dairou
Darrius
Kenshi (I lobe him...but for somereason I dont like his gameplay in this game that much)

i play as Li Mei alot. but when i need a break from her, i always go to Tanya, Ashrah and Mileena. I Also i rarely use Jade, Kira, and Sindel (only because i dont know how to use them that well). all the other male characters, i never touch them.

1)Havik: love being a Deadite
2)Kobra: like the kicks and the sticks

Characters that I feel most comfortable with.
- Sub-Zero
- Liu Kang
- Raiden
Characters that I am quite skilled with.
- Scorpion
- Noob Smoke
- Jade

My BEST characters:
Noob&Smoke
Scorpion
shujinko

oh man my best characters there are a shit load of them.
My BEST OF THE BEST!!!!
SCORPION!
KABAl.
havik
sub-zero
noob smoke
mileena
Baraka
raiden
liu kang
kobra
darrius
and dairou
GOOD WITH THEM BUT RARELY USE THEM!
Jade
lie mei
bo rai cho
sindel
and nightwol
so yea thats it.

I like to use a variety of characters
Baraka
Shujinko
Kobra (still learning with him)
Scorpion
Kenshi
Ermac
SubZero

I alternate between Ermac and Kira
I prefer Ermac because he does not has long boring dial-a-combo. But still Kira is interesting. I love to connect that death kiss for a free hit (when all their breakers are gone)
Currently learning Mileena.

1~sindel~love playing w/ her.. i make her look like a top character. .
2~tanya~now that i have found the cheese corner combo, i use her the most now.. if u dont know what i mean look at the sticked thread on top, titled high level gameplay.. because of this she is in the top tier along w/ the other cheesers of mk6.. the big 4.. lol
3~kabal~hook swords, love em..lol lots of skills needed..
4~ashrah~all 3 styles r great.. she is the 50/50 whore of mk6..
5~kenshi~awesome character, fun to play.. just stay in weapon stance..
6~hotaru~just started using him.. hes good! pop up, then 2nd stance 22 up 1.. easy and good damage.
ill pick mileena, raiden and nightwolf vs rookies online to give them a chance to win.. if u know what i mean..
ill also pick jinko and scorpion from time to time, just because they r good...

actually I never played the game, but i will instantly start with:
Sub-Zero, Shujinko, Scorpion, the super fighter fat ass with the muffler ... :)

Dairou, Kobra and Hotrau against the computer because the A.I is cheap, so I am cheap back.
Sub-Zero and Scorpion for chess Kombat as they are great for Grunts.
Puzzle Kombat I don't play much.
I like Kenshi and Kabal for the Konquest fights, if I get them.

I'm all Tanya.
Invisibilty>>Cannon Drills and Kobu Jutsu lows, gotta love that. And that corner throw combo: 60% ain't too bad at all . Plus, win or lose, you always got one of the most badass fatalities/hara kiris to use with her.

scorpion,mileena and ermac
that are my favo 's

Mileena and Sindel

1 - baraka
2 -scorpion
3- darius
4- kombra
5- li mei.

I love to use Tanya I think she is great also I use Scorpion and Mileena and Sub-zero.

MILEENA is #1!!!!!! I always use mileena, and its easy to beat Onaga with her.....but Ermac is also a lot of fun to use, and Kira...o yea Sindel and jade are cool too..........