Even though they are playable in every version afterward, in the original Street Fighter 2 players could not select them. M.Bison, Vega, Balrog, and Sagat made up the final four fights. The were all powerful and had an extra special character trait that made them stand out against the regular cast of fighters.
Balrog was a boxer who could only punch and not kick. He was very strong letting the player know that the difficulty was ramping up. In the Japanese version he was named M.Bison (a play on Mike Tyson's name) and had to be changed in America in order to avoid a lawsuit. Balrog clearly looks a lot like Tyson and takes almost all of his character design from him.
Vega was an extremely fast opponent. He could climb on the walls of his level and was the only fighter who had a weapon. He is the complete opposite of the Balrog fight beforehand trading in slow powerful punches for lightning fast arial manuevers. Since Vega was so different from any other fighter the difficulty ramped up again creating a great build to the climax of the game.
Sagat is the third fighter returning from Street Fighter 1. He was the boss of the original game and now stands as the penultimate boss. He has fireballs like Ryu and Ken but was stronger and because of his tall stature had a very long reach. He may have been a more tradition fighter but his attacks and special techniques made him a perfect boss.
M.Bison was the final boss of the game. He was incredibly strong and fast. He was given a military look making him seem more menacing and powerful. Since he was the last boss he was by far the hardest battle in the game, and beating him was a reward in itself. His main colour scheme is red so it is more than a coincidence that this colour was chosen for the final battle.