well...
i think noob saibot is the thoughest

MK1=Goro
MK2=Noob Saibot,Jade,Smoke(at least I can beat Kintaro and Shao Kahn)
MK3=?(Endurance Matches)
MK4=None
MKT=Everyone after the 3rd charcter.
MK5=Quan Chi on MAX
MKD=Noob-Smoke.

MK1- Shang Tsung: He kept being so friggin cheap.
MK2- Never Played It
MK3/UMK3/MKT- Endurance Matches
MK4- Never Played It
MKDA- Moloch
MKD- Noob-Smoke

MK1 - evidently Shang Tsung (Reptile is a piece of cake)
MK2 - Shao Kahn, but Noob Saibot and Smoke (both use the same AI patterns, so they are different only in looks)
MK3 - meh, Shao Kahn
MK4 - meh, noone even on ultimate
MKDA - tough call, 100%ed Shangh Tsung and Quan Chi
MKD - Noob-Smoke maxed, Onaga isn't as close though

MK1 - Goro
MK2 - Kintaro and Shao Kahn
MK3 - Motaro
MK4 - Goro (really easy)
MKDA - Never played it
MKD - Noob Smoke (Online players are the hardest)

MK1-last endurance match
MK2-jade
MK3-hmmm, nobody =)
UMK3-shoa kahn (as long as your not ermac, scorpion or h.smoke)
MK4-easy all around
MKM:SZ-that big robot on the prison stage
MKDA-dont play it
MKD-noob/smoke

MK1- Nobody! How can any of you see them as difficult?
MKII- Smoke hands down (he moves so freakin' FAST!)
MK3- Motaro (hey what can I say? I use projectiles ALOT)
MK:MSZ- Definitely the Fire God (idk why but he just gave me alot of trouble!)
MK4- You're kidding me right?
MK:DA- Quan Chi as the final boss
MK:D- Onaga....the one who developed that boss needs to be tortured in the absolute cruelest way possible....He doesn't distinguish between high and low blocking (so alternating does shit), he's fast (big down side there), he's cheap as hell (always seems to get under my defenses...), and he has that horribly cheap (and damaging) grapple move that always screws you up JUST as you're about to break a kamidogu....I hate Onaga....so fucking much...

noob-smoke isnt tough, they jus fuckin cheap with all there switch out moves

Noob-Smoke I find are very easy especially in MAX mode. You eventually learn their patters. I find they are way easy whenever I am Kabal as he is too fast for Smoke.

The hardest sub boss is Motaro. Cheesy bitch.
The hardest boss is Shao Khan in UMK3. One mistake and you lose half you life. After that comes Onaga on MAX. He is a cheesy bitch as well.

"No kicking" Well then stop kicking me to death whenever I play Kabal

I don't know if its just me but i thik Shau Kahn in MK Trilogy is frikin had.