The four playable bosses from SF2 special editions: championship, turbo, super, etc. For different reasons for each one, but here they are. I'm going by the UK / US names here (they swapped them for some of the international ports, so what UK players know as Vega was actually Balrog, Balrog was M Bison (M Bison = Mike Tyson, d'oh!) and M Bison was Balrog. Sagat was still Sagat.
So here's what was wrong with the four bosses.
Balrog: just pathetic against the computer opponents. His moves were so unbalanced: his straight hard punch, for example, could easily finish some characters off (particularly the ones who came aggressively forward, who tended to be the easiest anyway) but his slow speed and lack of decent special moves made him really weak against any opponent who didn't try and come after you. Plus he had no kicks or wrestling moves (just the crappy headbutt hold) - where's the variety?
Vega: game-breakingly powerful. When I finished SF2 with no cheats and sixteen double-perfects in a row, this is the guy I used to do it. It wasn't even his special moves; he could just claw every computer opponent to death at range, and very few of them could pose a challenge to him if he did, even on the very hardest difficulty. He was actually great in multiplayer, because you had to use a lot of strategy to beat a decent Vega player (typically feinting to make him try one of his quick jumps at you, then following it up with an uppercut) but that doesn't diminish how much he broke the single-player.
Sagat: again, his power at any range was pretty game-breaking, especially in multiplayer: if you really, really want somebody to hate you, follow a fast low fireball with a slow high one and watch them fail. Against computer opponents, he's not as great as Vega, but he's still comparatively easy to exploit in order to just take out opponents before they can even get close (or failing that, grab them and hurl them across the screen: Sagat's throw was brutal.)
Bison: I'd include him because he was a disappointment! The Bison you faced at the end of the original Street Fighter 2 was BRUTAL and hard as nails, even on the lower difficulties. He was, for example, invincible when he threw those long kicks at you. To make him a playable character, they nerfed the heck out of him, and it became more and more obvious as the SF2 games went on. In multiplayer especially: if you played against someone who was even remotely good with an uppercut like Sagat's or Ryu / Ken's Dragon Punches, you'd have zero chance of beating them with Bison. This was a character whose every single move could be detected and countered very easily with one or two specific moves from most of the other characters. Head-stomp, psycho crush, scissor kick, or straight-forward aggression - all of them were ridiculously easy to anticipate and to counter.