Interesting question. I'd have to say for me, right now, probably a Redline Monocog Flight 29er. I have road covered (Trek 7.3FX), geared mountain covered (Specialized Hardrock) but would love a SS. I rode a Monocog and came back gigglng. Too much fun, but definitely not for everyone.
As far as a general purpose bike, that's a very hard question. I had a good amount in my bank account and decided it was time for a good commuter. I looked at quite a few but it came down to Volpe vs Trek FX. Ironically, the bike I always had lusted over (Volpe) lost out to the Trek FX. Had the Volpe been a '07 model, I think things would have been different.
My suggestion is to just start riding something at the shop, then compare it to others. Where something = whatever the first bike the salesperson recommends. Shop around extensively, then buy the bike that just feels right.