Candy is the best pure trainer. Period.
Mentor would be among the best supporters, if not THE best. Tho some decks can run alternatives such as Lanette's or Fan Club, Mentor is the more wide-spread one.
As for Stadium, I say Power Tree on one side, tho I'm taking a liking to Holon Circle as well.
Tools...while Cessation Crystal is influential, I wouldn't call it the best. For that, I'd opt for Strength Charm, just to get to that tidbit of extra damage you need.