For athlon XPs the nforce2 is the BEST chipset on the market bar none.
For athlon 64s & FXs the nforce3 and the via chipset are very very close - i cant say which is faster, some prefer one, some the other.
As for P4s it would seem that the intel 875 is the best chipset avalible. but dont quote me on that one.
hope that helps
Steg