The Z-Fighters vs Vegeta. That battle never gets old. Goku vs Cell is a close second and Goku vs Freeza is close third. Honorable mentions go to Goku vs Piccolo Jr and Goku vs Jackie Chun.
As far as the movies go, I'd say Goku and Vegeta vs Metal Cooler was the best battle.
Best Fight: Goku VS Frieza. Or. Goku VS Vegeta or in movie 12 the 2 minutes of badassery we call SSJ Gogeta VS Janemba.
Best moment: Gohan and cell's Kamehameha battle in the anime. Goku going SSJ first time. Gohan going SSJ2 first time and moping the floor with the cell juniors.
Best Fight (Anime/Manga): Goku vs. Frieza
Best Moment (Anime/Manga): Goku's 1st Super Saiyan Transformation
Best Fight (Movie): Hard to pick for me. I liked the first 3 movie's action scenes for how well choreographed they were:
Goku vs. Nicky, Sansho, & Ginger
Goku vs. Possessed Piccolo
Goku vs. Crusher Corps
Goku vs. Byrrus kind of went back to the martial arts style of fighting the series originally had but still kept the modern ki blast spamming
I liked Goku and Frieza's rematch but nowhere near as good as their first bout.
Best Movie Moment: Eh I'd either say Goku turning Super Saiyan vs. Cooler or Gohan's final clash with Bojack and his gang.
Best FIGHT or best fight SCENE?...
Fight in general is WAY too hard to choose. :/
Scene: Again to many, but that time I stopped Trunks' sword with my finger looked pretty funny. XD
Best Fight - Either Tenshinhan's fight with Jackie Chun or Goku. Definitely the latter manga-wise.
Best Moment - Piccolo sacrificing himself against Nappa. Debatably the last death to have meaning in the series due to it seeming as though the DBs were no longer an option.
Outside of battle scenes, my vote would go for the Ginyu Tokusentai's entrance in Kai.
I like the fight scenes from earlier in Dragon Ball Z where martial arts still mattered and there was better choreography. After Super Saiyan most battles had too many ki blasts and too much flying. Dead Zone has some good choreagraphy.