MKII for the Genesis plays faster than the SNES version, at the loss of color, sounds, animation frames in win poses, etc. the 32x version is the closest youre going to get to the arcade version. the music in both versions, however is amazeballs if you prefer genesis music to SNES music.
Street Fighter II is very playable on the genesis. it looks and sounds better on the SNES by leaps and bounds, but playability-wise the two are on even ground.
ESWAT is a great arcade port, but sega changed a lot bringing it home, almost making an entirely different game. The did the same with Shadow Dancer, and Treco gave the same treatment to Atomic Robo-Kid (which is a great platformer itself.)
A lot of the Namco ports were great. I LOVE Burning Force and Phelios on the Genesis.
Pick up Grind Stormer/V-Five if you can. AWESOME vertical shooter that lets you change the play mechanics from the option menu.
The superscaler ports (AFII, OR, etc) leave a LOT to be desired. Theyre too slow/choppy for me to get a lot of enjoyment out of them. The old girl just couldn't keep up with the speed those games needed. get the 32x version of AFII NOW.