Depends on how much you want to spend really:
Higher-end MU units sound amazing if you have the audio setup to go with it. With a good DAC and Amp, the digital output on my MU2000 is simply unmatched. The 2000's sequencer is also a really useful feature if you want get into MIDI editing.
Korg NS5R. I got mine for $50, and I quite like this module. It's samples suffer a bit from compression, but once you get used to it's sound, it sounds quite good. This is a very quirky module that takes some time to get used to. Turning the Effect Follow option on (it defaults to off) makes the instruments sound much better, which really helps Descent, but can make other games sound terrible. You can even instruct the module to ignore GM/GS/XG reset messages, allowing you use exactly the instruments you want. You can freely mix all the sound banks as you like, this module is a tweakers dream. And the ability to throw a Dreamblaster inside helps too.
Akai SG01k. Very underrated and overlooked module. Amazing basses. Instruments are unbalanced, orchestral stuff sounds terrible. But this is my favorite module for "in your face" games like Super Street Fighter 2, Doom, and Rise of the Triad. Sounds completely different from all the Japanese modules I own.
https://youtu.be/OhPQIPKcOhA?t=16m37s
Modules: CM-64, CM-500, SC-55MkII, SC-88 Pro, SY22, TG100, MU2000EX, PLG100-SG, PLG150-DR, PLG150-AN, SG01k, NS5R, GZ-50M, SN-U110-07, SN-U110-10, Pocket Studio 5, DreamBlaster S2, X2, McFly, E-Wave, QWave, CrystalBlaster C2, Yucatan FX, BeepBlaster