For overclocking: Abit KT7 or Asus A7V are the best choises, and pretty much equally good according to many reviews. Performance of both of these boards is among the highest of any Socket-A mobos.
For non-overclocker: Microstar K7Tpro. This is propably the most stable and trouble-free Socket-A motherboard out there, and it's very affordable (little over $100). A couple of Anand's WWW-servers are running on these boards, that should be a testament to their stability.