Being the Reach/H4 youth that I am...
Sandman's M1 was pretty spectacular. I think that was the montage that first got me going for clips in Reach.
https://www.youtube.com/watch?v=WsNFYbOp5d4
I started out hating Pichufrenzy's Halo Montage 2 but the more I've watched it the more it grows on me. Soundtrack is happy and clips are the most unique I've seen.
https://www.youtube.com/watch?v=9yzWtlDd22M
I could go on, but I don't think very many of you care about Reach/H4 tages, so I'll stop there.