Other strong candidates naturally include GO!! (also by FLOW), A Cruel Angel's Thesis, and Guren no Yumiya (which is still good regardless of the quality of the media it was attached to)
Agreed on all counts. Go!!!, while being in an anime I don't really care much for anymore, definitely has a special spot in my heart, what with being my favorite Naruto OP at the peak of my interest (before taking a nosedive). Really brings back a lot of happy memories. God Code Geass was a blast, way back when.
Wait no, it's tied with "Seishun Kyosokyoku" for my favorite. The Sasuke Retrieval Arc hit a lot of the notes I really liked. Now I'm just thinking of Nick's endearing handle being "RockLeeSmile" and how I can totally get that. Amusingly simple, but good.
Haruka Kanata will always have a special place for similar reasons, as I liked it a little better than GO!! This goes double for Asterisk by Orange Range as it is actually a GREAT SONG
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It's musically very similar to COLORS, but it holds a special place in my heart because it's used for the first arc of Eureka Seven.
it goes without saying that this is the actual best, of course