There's a lot of good ones out there, but two come to mind immediately.
First, there is Hentai Taren no susume
, which is basically every insult Iori has said strung together in non-stop, pure awesomeness. Well, I suppose I shouldn't call it a video. Nevertheless, I love it. Great to listen to before bedtime!
Secondly, I'm pretty fond of Danketsu
, since it manages to capture the entire cast's personalities in one song.
As far as dance songs go, I confess...