The Behringher BSY600 is a clone or copy of the Boss SYB5, although some say it has slightly better tracking. It's a bit cheaper too. The Boss is notorious for its tricky tracking. Both have expression pedal input for filter swells.
The EHX Bass Micro Synth is a really fun synth pedal - it has sliders for all the controls, so you can experiment easily. It generates its 'waveforms' with an octave and distortion effects, so the result is quite gritty, can be good for 70s Moog sounds. Has no presets. The old big box 12v one is supposed to sound better than the new small 9v one.
You might also want to look at:
Korg G5 - uses waveshaping to change your bass's tone to something synth-like, so tracking is good. Has nice vowel filters and presets, but is big unit. Discontinued.
Markbass Super Synth - fat sounding digital pedal with presets. You need a computer to edit all the parameters of the presets. Has an octaver effect too, synth only does sawtooth waveforms.
Akai SB1 Deep Impact - discontinued digital pedal, wide range of different waveforms including more comtemporary synth sounds. Simple editing of presets. Expensive second hand.
Chunk Systems Octavius Squeezer - digitally controlled analog VCO & VCF, very sophisticated editing (too complex for some people), not fat enough sounding for my ears due to only one oscillator.
With all these pedals, you will need to adapt your playing style, or your basslines, to get good results. They all have their tracking foibles.
Re. Legs, I can't think of a single pedal that will nail this sound. The sequencer element you will have to get by repeatedly playing the note, or maybe using a tremelo pedal before the synth. The waveform is square/pulse which rules out some of the pedals above.
You can go a long way with separate pedals - I know you want to add just one. But you could add an octave pedal (MXR Bass Octave Deluxe or second hand Boss OC2), a distortion pedal you may already have on your board, into the filter of your Bass Synth Wah - will give you a very tweakable synth sound.