Been through quite a few personally.
Groove Regulator - brilliant; should definitely be on your 'must try' list.
DOD FX 25 (and - much less versatile, and you'll need to spend some time learning to play them to get the most out of them. The FX25 has a real volume spike towards the bottom of its sweep, so best used of fast notes or in conjunction with a compressor.
Q-Tron - Funky filters, but have a tendency to produce said volume spikes.
Bassballs - not one that I got on with personally. Kind of fun to play with, but not the classic 'quacking' sound that most players associate with filters.
Various Wah pedals - you'll lose mobility as you need to be working the pedal whenever you need a filter. Good for sweeping over long, repetitive passages. Also not good if oyu fall over a lot when stood on one leg...
Hope this helps.