DB
Goku vs Bulma's Car. No explanation necessary.
Kuririn vs Bacterian. Gross humor without the prepubescent peeing seen in DBZ!
Goku vs Tao Pie Pie, 2nd time. When Goku gets serious, he really does kick ass.
Goku vs Grandpa Gohan. Comic. The fact that the two are more or less equal really made me wonder just how powerful Goku's grandpappy really was. It also makes the battle a lot more meaningful, seeing as how Goku was at the height of his power (at the time).
DBZ
Goku vs Vegita, first time. Blablabla Vegita is cool blabla Kaioken blablabla earth shattering violence.
Goku (regular) vs Freeza. Comic version. More of a playful bout than a serious battle, really. Freeza is more excited about fighting an opponent who's actually strong as opposed to seriously dealing with enemies that only piss him off. Freeza takes his time with beating the stuffing out of Goku when he gets the upper hand, as well, earning him style points.
SSj Vegita vs Android 18. Vegita shows off his strength for about half the fight. We then see what has to be one of the best villain-driven beatdowns in Dragon Ball, ever. If there's one thing Android 18 can do, it's shove Vegita's pride right up his ass.
Goku vs Majin Vegita. A well-animated spectacle in which the two basically beat the crap out of each other in a truly painful way. (Of course, that filler bit with Vegita driving Goku towards the stalactite is just plain silly...)
DBGT
Goku, Trunks, Pan, and Giru vs the Bonpara Bros. I love the Bonpara Bros. They're the most original villain to come out of DBGT, and I really do wish we saw more of them beyond appearances in about three or four episodes. It's also one of the more original ways we've seen Goku and co get the stuffing beaten out of them. One can only wonder how Vegita would've taken it...!
Goku vs Bebi Vegita (on Earth). Goku gets the crap beaten out of him in a very, very painful fashion. This was during the time in DBGT where Goku's level of power made a bit more sense in comparison to everything else (unlike later on where his base form suddenly becomes the most powerful thing ever).
"I am an Android from the future, destined to absorb 17 and 18. ~<3"