I'll also go with Cell.
Gohan says "And here I'm supposed to have powered up considerably since then!"
In what way does that translate to "I AM more powerful than I was before!" If anything, Gohan is demonstrating that he's NOT as powerful as was. "I'm SUPPOSED to have powered up considerably since then." That sounds more like him admitting that he hasn't been training at all-- which is consistent with what Vegeta says at the Martial Arts Tournament. I know the timelines aren't exact, but it's already been stated that Gohan's power has dropped.
Hell, when SSJ2 Goku and SSJ2 Majin Vegeta are fighting, Vegeta notes that he's impressed that he's stronger than Gohan was as a Super Saiyan 2. Vegeta stresses that because prior to SSJ2 Goku and SSJ2 Vegeta, SSJ2 Cell Games Gohan was THE strongest Super Saiyan 2. You COULD make the argument that the Anime ignores the movies for the most part, but they're still considered to be the same continuity-- so that doesn't hold up.
Additionally, whether or not Gohan was a full-on Super Saiyan 2 is still a gray area that's much unsettled in the Dragon Ball community. While the guide-books support him being a Super Saiyan 2, his appearance in the movie is as a Super Saiyan 1. But hey, let's assume he COULD transform into a Super Saiyan 2 at the time: what makes you think he was a SSJ2 while he was firing his Kamehameha? He was getting the life crushed out of him by Broly and was undeniably weakened from the fight, so we can't even say for CERTAIN that he was a SSJ2 when he fired the Kamehameha.
But for the sake of argument, let's say he WAS a Super Saiyan 2 when he fired it. Broly was taking on a Kamehameha from an extremely weakened Super Saiyan 2. THEN he was also battling Goten's Kamehameha. Goten is a regular Super Saiyan, whom we don't even fully understand how strong he is. He's probably around the same strength level as SSJ Frieza Saga Goku, AND he was weakened. So it's hardly going to help at all in a beam struggle. Then you have Goku flying down to help them. Whether or not Goku's Kamehameha was actually there is STILL a gray area. We know Broly, Gohan and Goten all SAW him, but do we know if it was just an apparition of Goku that was sent with the Dragon Balls?
But I'll tell you what, I'm SO confident that Super Perfect Cell curbstomps this Broly, I'll give you a couple benefits for the sake of argument:
Let's assume Goku was physically there as a Super Saiyan, Gohan was a Super Saiyan 2, AND Goten was a little bit stronger than SSJ Frieza Saga Goku. Even so, LOOK AT WHAT IT TOOK TO BEAT CELL:
-A stronger SSJ2 Gohan (granted, his power was cut in half)
-The REST OF THE Z-FIGHTERS who at this point also consists of Piccolo (who's WAYYY stronger than SSJ Goten) AND Tien (who's powerful enough to hold off Semi-Perfect Cell, clearly ALSO far more powerful than SSJ Goten), Krillin, Yamcha, etc ALL FIRING KI BLASTS DIRECTLY AT CELL (and they STILL get blown away by his Ki without him consciously trying to blow them away)
-AND Vegeta (his SSJ form at this point should be stronger than his Super Vegeta form) firing a powerful Ki attack (not a wimpy ball of Ki like Trunks did) DIRECTLY AT CELL to distract him.
Oh, and mind you, it wasn't all of their sheer power that killed Cell. It was a powerfully distracting Ki Blast from Vegeta that did the trick. What distracted Broly/caused him to slip up again? Oh yeah, a wimpy little Ki ball that skid OFF his own Ki blast. It didn't even hit Broly, if I recall correctly.
Cell was simply taking on FAR more power than Broly was.
