This is hard to answer because "playable" means different things to different people.
Fears and Alien Breed 3D both runs decent on an 030/25 and well on an 030/50.
Gloom about the same.
Breathless is playable if you tweak the screen size on a 50 mhz, as is Doom, but it won't be that smooth.
Genetic Species runs pretty decent on 50 mhz as well. Gotta have more than 8 megs of RAM for sound though, as far as I can tell.