Hai friends

To everything (turn, turn, turn)
There is a season (turn, turn, turn)
And a time for every purpose . . .

OK, seriously, if you mean for desktop PCs, there is really only one answer.

But if you also specify a "For what?", you might get a variety of useful answers.
 
It depends on how much you are willing to spend...
The current fastest - i7 975 - $1000
And the low cost best - E5200/ 7750 - $70
So if you have a budget, then for that we could suggest the best...