It's worth bearing in mind which pedals are band pass, which are low pass and which do both.
I love the MXR but it only does band pass. Luckily it has a dry knob, otherwise it'd be pretty useless. If you're looking at bandpass filters, make sure they have either a blend or dry knob. When using lowpass this is less of an issue and I usually run LP fully wet anyway.
The Q-tron is good too. It has LP, BP and HP but also has a MIX setting which is bandpass mixed with dry (though you can't set the level of mix).
The Q-balls Enigma is also pretty damn good. It does most (if not all) of the stuff the Q-Tron does but has a few more knobs to tweak. It also has a built-in distortion channel which yields some dirty synth tones.
Check out the Proton, Fwonkbeta, Moogerfooger MF-101, Emma Discumbobulator, Keeley Neutrino, Mu-Tron III (and its variants) if you can find one.
I'm still working my way through a bunch of envelope filters.