It depends on what you want out of Goku, if you want a character that is simple to master I suggest SSJ Goku. If you want a good anchor to use with the new limit break I suggest Base Goku (who is my personal favourite), but you may need to altar your team to use him at his full potential. Goku GT is really good since he can easily extend combo's and he's very fast, but it might take a bit more effort to get used to his play style. Goku Blue plays similar to SSJ Goku and is usually a good choice if you want to try and get your opponent out of a block-string. And Goku Black is just an all around character, he's easy to input in any team due to his supers. So it's really all up to what you want out of your team. I hope you'll be able to figure out what Goku you wanna use. Good Luck!