Reaper makes the best mini's for D&D.
I-Kore makes excellent Sci-Fi mini's (thier VOID line).
The Crocidile Games Aegyptus miniatures are just plain cool, and I use the Rackham Confrontation mini's for some things, but they are a little too pricey to start a large collection.
And for Wierd Wars, I actually found a bag of "green army men" that are almost exactly 28mm scale! I painted up four of them, and they look great. (and cost about 10 cents a piece).