My vote's for Afco's on a racecar. Some of the high-end $450 Konis are better, there are others like Penske - but these are really on the top shelf for serious teams. Don't use Aldan's if possible. They still use the old Gabriel innards - Basically, cheap junk street rod shocks.
Strange and Varishock are both knock-off's of the QA1 - but they do all work nice - one thing to keep in mind - The AFCO's, on top of being double adjustable, use circle track-style valving, so you can revalve(or have it done) anyway you want to - if you get REALLY picky.
QA1's can not be customized internally, but they're a good shock for all but Warren Johnson
The difference is the AFCO's are $349ea and the QA1 doubles are about $70 cheaper a piece.