I've tried a ton of filters:
3Leaf Proton, Groove Regulator, GR2, Wonderlove
EHX Bassballs, Enigma Q-Balls, Mini Q-Tron (not the newer Micro), original Q-Tron
Keeley Neutrino
Beigel Tru-Tron 3X
WMD Fatman
MXR Bass Envelope Filter
Boss FT-2
Tonefactor 442
Zoom Multi-FX (B1Xon, B3)
My favourites are the two old Q-Trons. The original sounds best with playing low down and is more versatile, but the Mini has a certain something that makes it extra funky even though it doesn't sweep as low. These can be picked up used very cheaply, only downside is their size (the Mini is not very mini!) and power requirements (Mini is 9v but has a funky socket, original is 24v).
2nd fave is the TruTron 3X. Apparently (I've not tried one) it does everything the original Mutron III does plus more (being designed by the same guy, who also incidentally did the original Q-Trons). However since I only play these pedals occasionally for fun and it's a pretty expensive unit, I'm thinking of moving it on.
3rd for me is the GR2 and Groove Regulator (GR2 just edges it). Amazing versatility, great envelope action, very fat sounding but just not as wet sounding as the 'Trons.
4th is the MXR. Not as fat since it's a bandpass mixed with clean, but it is the best filter for slap and can get some very unique sounds.
Of course other people could list the total reverse order to me as everyone has different preferences, filters are a very personal effect and respond uniquely to different playing styles and basses!
The ones I have not tried but really want to are the Chunk Systems 00Funk (hard to come by though) and Source Audio Manta. The Manta has so many filter models built in I'd be amazed if you didn't find one you liked. The Zoom units also do a very nice model of the Q-Tron and a couple of others and are cheap as chips (especially if you buy my used B1xon!).