Cha-La-Head-Cha-La by a pretty wide margin, though either We Gotta Power or Dan Dan Kokoro Hikareteku come to mind as runner ups for openings. Including video games, Ore wa Tokoton Tomaranai and Kiseki no Hono yo Moeagare are on it's level though.
For endings, all the GT Endings are equal picks for me, unless counting movie endings, in which case Saikyo wa no Fusion wins by a landslide. Whilst I used to dislike the 2nd ending of Kai's Boo Arc, it has grown on me enough to give it an honourable mention due to it's beat and Ieri's voice being pretty good, though the Engrish is a lot less structured than in most of Kageyama's songs which holds it back a lot.