Well, according to theoretical <A HREF="http://www.tweakers.com.au/articles/cpu_mem/ddr_roundup/page1.asp" target="_new">benches</A>, the Crucial PC2700 and Kingmax PC2700 will offer the most performance, and given the reputation of those companies, I'd go with them. In fact, Corsair XMS3000 performs relatively worse than the all the other sticks tested, all the way from 133/133 to 166/166. Granted, you're promised 187MHz out of the Corsair, but I don't think it's worth it. Since all the other sticks tested were able to handle the most agressive memory timings at 166/166, it's probably safe to say that you could get the others to 187MHz or even 200Mhz (though that's probably pushing it) with less agressive timings and good cooling.
If you're looking to overclock with the best performance, I'd say Crucial PC2700 or Kingmax PC2700.
If you're looking for the most headroom that removes the RAM as a bottleneck to extreme overclocking, the Corsair PC3000 would be the best choice, as it is rated 187MHz@CAS2, and not the PC3200, as the CAS of 2.5 seriously downgrades the performance, and it shows that the stick is already near the end of its overclocking ability. At least with the PC3000, you can ease off on the memory timings to squeeze out a few more MHz on the FSB.
Disclaimer: Since these are theoretical benchmarks, and the results spread is less then 10% (actually no more than 4% for most of the tests), it is most likely impossible for a human to preceive the differences between the Kingmax or Crucial and Corsair. Also, since these samples were supplied by the companies and not purchased through regular channels, they are possibly hand-picked sticks, and not representative of the actual performance of the product. In the end, you should make your decision on what you want: a high performing system overclocked to get more performance, or bragging rights to the highest extreme overclock possible on a certain setup. Both are valid choices, though I lean to the performance, and therefore the Kingmax and Crucial.
-SammyBoy