Well openings and endings are a different matter. I have a billion fav openings, but my top 5 are probably:
My Soul, Your Beats! (Angel Beats)
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Sailor Suit Fuku (Lucky Star)
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Red Fraction (Black Lagoon)
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Kyoumu Densen (Another)
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Küsö Mesorogiwi (Mirai Nikki)
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And remaining unlinked on my top list are various Bleach, Code Geass, and the Clannad OPs, as well as Tank from Cowboy Bebop, Lilium from Elfen Lied, and Higurashi No Naku Koro Ni OP.
As for endings, well, these are amazing:
Still Doll (Vampire Knight)
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Brave Song (Angel Beats)
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Mad Surfer (Bleach)
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Dango Daikozoku (Clannad)
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AND THE BESTEST ONE IS MARUKAITE CHIKYUU!!!