Bra, a 10 (or 11, she has to be about Pan's age though, and Pan is supposed to be about 10) year old girl who dresses like she's 15, manages to convince Vegeta, her own father, to shave his hideous mustache. The Prince Of All Saiyans got ordered around by his 10 year old daughter. Later, he's seen taking her shopping. A 10 year old girl has Vegeta under her thumb. That's badass.
Goku transforming into a Golden Great Ape for he first time. Many thought of the time he first transformed into a regular Great Ape as rather scary, but this transformation was seen as epic.
Even better was Goku transforming into a Super Saiyan 4 for the first time. The Funimation version of this moment comes complete with a badass Voice of the Legion effect as Shane Ray (Giant Ape Goku) and Sean Schemmel (Adult Goku) scream together.
He lets Baby hammer away at him and even nail him with the Revenge Death Ball...just to show Baby he's basically powerless against him. Then Goku fights back.
Dragon Ball GT had, let's face it, a lot of problems. Super Saiyan 4 Gogeta was not one of them. In fact, the entire shadow dragon saga was a significant improvement over the rest of GT.
You also have to hand it to Syn Shenron. Once he absorbed the power of the other Shadow Dragons, he became powerful enough to fight on equal footing with 2 Super Saiyon 4s!
The fight with Lood, where Goku's usual way of handling things doesn't work and instead requires a host of characters to work together in a massive Enemy Mine gambit to take him/it down; it's definitely one of the better parts (if not the absolute best part) of GT before it was retooled.
Mr. Satan, the guy may have lost his hair and is nothing power-wise compared to the heroes. But when he thinks everyone, including Pan, has been killed by Giant Ape Baby; he stands in front of the giant monster and starts screaming how he's going to kill Baby despite knowing he stands no chance.