I've tried a good few down the years, so here's my tuppenceworth.
A good generation is worth it's weight in gold, but for me the redtops are more consistent and nicer than the blue. Oak make a nice whistle in D, but sift thru loads to find a good one. Chieftains can be a pain, tuning wise. Susatos are rather plasticcy and vulgarly loud, though they do have their place. Overtons are beautiful if a little pricey. I wouldn't touch a Tony Dixon with a bargepole (maybe I'm spoilt having had an Overton for years). If money's not a problem, Mike Burke out of Murphysboro, Illinois makes absolutely fantastic whistles in a range of keys and materials (starts about $100). Believe me, these are seriously great whistles. Mike Grinter from Australia also makes great instruments (wooden) - check out Ciaran Tourish (Altan) who plays Grinter whistles. A word of advice, though, don't sacrifice tone for volume. If a whistle sounds loud AND crappy, everyone will hear how crappy it is. Good hunting!