I adhere to SSG > SSJ4, but I guess I could see why some fans might consider the latter to be the best. GT characters were seemingly leagues above anything that was seen in Z; even characters like Frieza, Cell, and Piccolo were all likely several tiers stronger than Buuhan and possibly even Vegetto. Heck, even SSJ Goten and Trunks were probably stronger than Ultimate Gohan (Buu arc), seeing as how well Goten (post-Baby) did against Gohan, who at that point had a base form that was presumably stronger than his Ultimate incarnation.
But regardless of all that, I still feel the top-tier of BoG and RoF is several notches stronger than the top-tier of GT. As for the final form, SSG definitely seems to fit the part best in my opinion.
As cool as SSJ3 was, I feel the comic relief aspect of the Buu arc hindered it's potential a bit. If anything, I was quite impressed by GT SSJ3 Goku (against Baby Vegeta).