I've also got the 127, and it's by far my most-used lens--quite versatile, and very sharp. Possibly even sharper is the 90--not long after getting mine, I did a full-length portrait of a friend, leaving a bit of room beneath his feet and above his head. Under magnification, the lettering on the button holding up his jeans was clearly legible and extremely well-defined--that's resolution!
I've also got the 50 and the 180, and they're no slouches either, particularly the 180. I went for the C versions because they are cheaper, they're generally nearly as good (95% or more) as the KL-L lenses, and they're cheaper to repair if the shutter goes out (an important consideration if buying second hand with no knowledge of the lens's provenance or prior usage).
Another lens that's not been mentioned so far is the 140 macro. It's also amazingly good--I've used it as a portrait lens, and with its close-range correction, it's very useful if you ever want to do closeups with your RB. And at under $350 at keh.com, they're quite a bargain compared to many 35 mm macro lenses. You'll of course need the extension tube(s) to get full advantage out of this lens.