I like the packs that come out around Christmas with red, green, and white. The white ones are a bit bland but I think red nerds may be my favorite with green a close second.
Sour nerds, when they can be found, are clearly the best, but again are so difficult to locate they almost cannot be ranked.
The true greatness from Christmas nerds, though, is that after Christmas they are usually heavily discounted. Last year I bought enough to last me through mid-summer for $10, and I am a man who loves my nerds.