Alright, it's Sagat or Akuma and the more you play them, the more you see it. I'll start with the obvious one
Sagat:
Fireballs: This is obvious his Tiger shots are very good, they travel fast and they are the most damaging normal fireballs (not countin EX/Akuma's red fireball) he can fire them high and low to catch you if you jump or if you try to go under them and the ex fireball will knock you back halfway across the screen to let him reset the fireball game. As far as fireballs go he's just too good.
Damage: again this is obvious a simple Tiger uppercut, FADC, F.HK, Ultra does 622 damage! That's well over half the average characters health and for Seth and Akuma they are almost dead, his Cr.fierce does 130 and if you get counter hit you can link uppercut or without counter hit a super. The only characters that hit harder are honda and Gief, and they have to work to get in for their damage, as Sagat people are happy to stay close. Add all this to a fast and damaging overhead he's amazingly good.
Anti-airs: His tiger uppercut, standing HK and and f.hk make him almost impossible to jump in on without a deathwish, add that to his fireball game he is one of the best zoning characters in the game.
Health: He's got an above average health bar and has above average stun, so even when you get in on him, you still have a lot of work still to do.
His only real drawback is lack of footsies, but all you need is one kara-uppercut and dash back then they are where you want them again.
Akuma:
Footsies: His footsies are insanely good, he has an amazing cr.mk which can be cancelled into a fireball possibly one of the best footsie tools in SFIV, his walk speed is decent allowing him to walk in and grab you without making it too obvious and easy to tech, You can't pin him into a corner and apply pressure as you either have to step back to punish a teleport or let him escape with it, letting him out of some situations which are deadly for other characters; don't forget about his air fireballs, they maintain pressure and keep you locked down even while he advances or retreats makes his zoning just to notch.
Damage/Stun: Too many people sleep on Akuma here his bread and butter combo does almost 500 damage and 620 stun, more than Zangief's stun headbut. His ultra while hard to hit does 600 damage making it one of the most powerful in the game and making you get scared when you are gambling against an Akuma with ultra.
Fireballs: His regular fireballs are nothing special, they are slightly slower than Ryu's but that's not where his strength lies, his red fireball hits you 3 times making it impossible to focus attack, it will eat an EX fireball and tag you with 150 damage. There's a risk because of the long recovery but its worth it from full screen where it's safe.
Mixups: This is where Akuma shines through, he can mix you up with cross up tatsus making it hard to guess where to block and eating 200 damage and a knockdown for guessing wrong he can reset you after a light tatsu to reset the damage scaling on a combo and get stupid damage in a short time. He can throw an air fireball on one side of you and chose which side to land on, making you guess which way to block the fireball and eating a fat combo if you guess wrong.
His only weakness is his health, even that isn't a huge drawback as he can still take beating, compared to Seth.