Lets see, Mine would have to be...
In no particular order,
Uninstall (Bokurano OP)
Aru Machi no Gunjou (Tekkon Kinkreet ED)
Crossing Days (Kurenai ED)
Split (Honey & Clover II ED)
TONIGHT TONIGHT TONIGHT (Bleach ED, Don't know which one.)
Kumikyouku Nico Nico Douga & Lucky Star Douga get special mention too, though.