I practically like all of the 4Kids ones, with my favourites - mainly for the nostalgia - being:
Unbeatable (specifically the long version)
I wanna be a Hero
Advanced Adventure (I don't understand it at all, but the tune is so dang epic, even instrumental)
Other songs that aren't openings I like are Double Trouble, Advanced Adventure as BGM, the Pokéraps and lots of the Wild Music remixes are pretty awesome.