According to Dragonball GT, Vegeta. However in GT Gohan never uses his "Mystic" powers which actually made him a lot more powerful than SSJ3 Goku in DBZ so stricty speaking gohan is capible of becoming more powerful than vegeta, but chooses not to or is incapible of doing so anymore. A possible reason for this is that Gohan is known for taking long breaks from his training in the latter parts of the series, whereas Vegeta is constantly improving.