I addressed Seth in another thread, so I guess I'll just copy paste it here:
Just a tip, Seth's AI is fairly abusable in SFIV. If you stay at long range from him, he'll spam sonic booms until you get close, and if you stay at close range he'll most likely throw you. The easiest way to beat him is just to stay at medium range, and 80% of the time he'll try to do a long punch (Dhalsim's one), just jump over this with a jumping attack, then go into a combo from there. At other times he'll try to use that vacuum move of his, but you can jump over that too. After you hit him with a combo, go back into medium range. Rinse and repeat, and suddenly he's the easiest character in the game to beat, even on hardest.
OT: Probably I-no from Guilty Gear, I remember raging hard when I was trying to beat her. Nu in Blazblue: Calamity Trigger is also suitably frustrating, but not on the level of I-no.