A lot of the Naruto openings, frankly. The series is part of the dreaded Shounen Jump Trio as well as the notorious animation issues the series had, but a lot of the openings have some of the best animation I've seen.
And yeah, I pretty much rate OPs based mostly on the quality of their animation, less so on their sheer awesomeness.
If I had to say, it'd be Naruto (not Shippuuden) OPs 2 and 4. I won't link anything because they're all crappy flipped OPs or have huge borders because the Shounen Jump Trio have huge copyright issues with their OPs on Youtube. There are a series of other OPs I like, but the fluidity and quality of the older series' OPs are much appreciated.
Edit: Speaking of well-animated OPs, FMA is also very good.
