Depends obviously on what you're looking to spend and if you're really trying to go light. Tough to beat the sub 3# carbon frames if you're trying to build a real, dedicated race bike. I have 2 Lynskey Ti SS frames, a Ridgeline and a Pro29. The Pro29 is a stiff racing machine but I've bikepacked on it as well so it's pretty darn multipurpose. That frame is a smidge over 4#. I'm actually selling it since I've decided I don't need 2 Ti frames. (2012 Lynskey medium Pro29 SL - Buy and Sell and Review Mountain Bikes and Accessories)
Another great Ti frame are the Vassago Optimus Ti's...built in PHX, they are solid, well build machines that will serve you well for years.
Another great Ti frame are the Vassago Optimus Ti's...built in PHX, they are solid, well build machines that will serve you well for years.