Cyrix is definitely the worst, but then again it's a nice cheap solution for the people that want a $300 computer.
The Thunderbird has by far the best architecture & potential, but is currently being hampered by it's motherboard & memory options. Once DDR Athlon boards hit the mainstream market, it will probably be the performance leader. Unless, of course, the P4 is already out in mass production.
The Pentium III would be my current choice for CPU leader. True, it's based on old architecture, but currently it has the best across the board performance & compatibility. Ever wonder why P3's & Xeons are pretty much the only choice for the server market?
Viper GTS