Well, my favourite opening is definitely Noragami's:
And for complete, adorable crack and beautiful, realistic and not-at-all-demonic dancing, this particular Hetalia ending:
https://www.youtube.com/watch?v=gnET1FFYuPQ
(sadly I can't find a video of Italy's graceful motion XD)
Doesn't help that these are the only two animes I regularly watch :3