Rozard's "Excel's Day Job," SpPanda's "Ever 'R's," and melezov's "To Be Excel" are my favorite Intros. Melezov's is the best because it's less than 1 second long
Personally I haven't done a real intro yet, I've only done outtros thus far, but I have fun with them, by putting in scenes that are related to the AMV. Whee.

I try not to spend an overly large amount of time doing them, though. 1 hour is the most I've spent on an outtro.