I'd say Omega Shenron. SSJ3 Goku was doing fairly well against Janemba and Hirudegarn was taken out by SSJ3 Goku. Omega was toying with 2 SSJ4's. I haven't seen the Hirudegarn movie in a while so I could be overlooking something.
@LHWKnight: So acording to you it goes like this: Goku SSJ3>Fat Janemba>>>Omega Shenron>>>Goku SSJ4>>>Goku SSJ3>>>Goku SSJ2>>Goku SSJ?
Goku GT at base level solo'd both Cell and Frieza like it was nothing witch puts his power at SSJ2 at the very least. Also at base lvl he fought Rildo who acording to Goku is Buu lvl. Now add all the SSJ transformations on top of that and that power doesn't even come close to Omega Shenron who while depowered (lost the 4 star ball) solo'd both Vegeta and Goku SSJ4. It took an Gogeta SSJ4 to stand up against the guy.
I'm done with Animevice. To much GT hate.