I am going for a 4 day trip in Zürich and I would like to bike quite a lot. Though biking is quite advertised, I failed to find a decent bike rental shop in the city. Would highly appreciate some tips!
Not sure what your requirements are (and as others already mentioned the "better" models are probably mainly available in mountain resorts), but there are at least some shops in Zurich where you can rent mountain bikes (sites in German only)
It looks like Spinlister has a few listings, though I'm not sure if any are exactly what you're looking for or how far out of town you'd be willing to travel to do the pickup.