Whilst I have the bass flyrig, GT-10b and the Zoom B3, I would say the POD 500x is by far and away the best.
It excels for drives, comps and EQ's. The octave is the best modelled octave down I've tried. The modulation's are great, as are the verbs and delays. Even the synth effects are pretty usable with a bit of tweaking. It did take me a good while to dial in the filters how I liked. I would also say the Bass model pack is essential too and very good.
I'm consistently surprised how quickly I can get the sound I'm after using the Edit software, and with a bit of practice how close you can get to the real thing when A/B'd
The Flyrig is great for is portability. I can cover most things with this pedal alone.
The GT-10b is a phenomenaly powerful effects processor. If you need to be able to control multiple effects and assign things all over the place it's the one for you. It's tweakability is unparalleled.
The Zoom B3 is amazing for the money. It has top class modelling and effects. E.g. It has my 2nd favourite octave model (after the 500x) and the synths rival the classics. But I do find it a little clunky in terms of usuabillity on a gig - switching between patches and individual effects is a bit tap dancy.