Gotenks has it hands down. Teen Gohan's best accomplishment was fighting, and defeating, Cell. After the Cell Saga, Cell was pretty easy to beat; Pikkon literally gives him a backhand and he was defeated. After defeating Cell, King Cold, Frieza, and a numerous amount of legendary warriors from the Other World Tournament, he was defeated by Goku.
Then there's Gotenks. Gotenks had fought equally against Super Buu, a powered up form of Majin Buu. Majin Buu's first form was his weakest due to the goodness in his heart competing with the evil that is Buu. Majin Buu's fat form had fought against Goku, and while he was in SSJ3, was able to give Fat Buu a serious beating until his time went up. Once Buu powered up to Super Buu, only Gotenks could stop him, holding his own in SSJ, and taking advantage while in SSJ3 like Goku.
The most key part in this is seeing Super Buu fighting Gohan early on before getting his Mystic training, he was brutalized and left for dead. While it was stated he was weaker since the Cell Saga, any marginal power gain wouldn't have made a difference. Considering Super Buu is much stronger than Cell, and since Gotenks was able to fight on equal terms with Super Buu and Gohan couldn't, Gotenks is therefore much stronger than Teen Gohan.