Peashooter and Grey Matter are characters that can hold their own, and will continue to be very valuable support characters even when you're at the Diamond level. I've heard good things about Graveyard Shift too on that front, but I haven't gotten a chance to test that out myself.
Doublicious, Purrfect Dark and Wrestler X are offensive characters I've taken to Diamond Gold and Gold respectively, and I don't regret any of those. All have strong utility, and Doublicious in particular has taken down people with twice her fight score with ease at times. I maxed out her Chaos Marquee Ability though, just for reference, so that's more about her ability to steal buffs and apply crazy debuffs than raw stats.
I've heard good things about Toad Warrior too, but I haven't had the chance to test that either.