Any quality CF or Ti frame will give you an excellent ride. I ride 6/4 Ti. Generally, 6/4 is a little lighter and stiffer than 3/2.5. I love the quality of the ride and the lightness of the frame (sub 2 lb). There are many grades of CF as well. I don't think you can go wrong with a good frame from Trek, LeMond, Giant, BMC, etc. I think the new Kestrel Evoke SL is a killer frame.

Like karesz3 said, get one that fits the type of riding you want to do. Go down to some LBS-s and test ride a few. Try to test bikes that have comparable builds on them, specifically the wheels, as a change in wheels can impart a completely different "ride" and what you want to compare are the frames.

Best.