High quality pellets and a varied diet of other foods- frozen or freeze-dried krill, brine shrimp, blood worms, etc, fish fillet, fresh shrimp, live foods (no feeder fish) and various pellets.
I have a GT, and I feed him Hikari cichlid gold, omega one shrimp pellets, frozen brine shrimp, and earthworms. (He's four inches right now, and I'll probably get krill as he gets larger)
Hikari is probably the most popular, because it is good quality, easily acquired, and relatively cheap. I've also heard good reviews of NLS, Omega one, and Dainichi.
Also I hear great reviews about hikari sinking carnivore pellets. I was going to get some the other day, but there weren't any in stock.