Bleach
Dexter's OP was a nostalgia blast, but so was Bleach kind of (made me nostalgic for a [brief] period of time when the show wasn't terrible). Anyway, more impressive visuals and I liked the song better.
True Blood
HIMYM has a nice opening sequence, but it's way too short to be of consideration sorry. And True Blood has the better song and impressive visuals to boot.
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Courtesy of fr0q - http://img.imgcake.com/gensagajpgqe.jpg
Punny worked hard on this! - http://img.imgcake.com/Punny/Gennypngas.png