^ too many things to list:
http://super-smash-bros.wikia.com/wiki/Mr._Game_&_Watch_(SSBB)
I really don't understand questions like these.... as there's no one character to choose, regardless of who your main is...
Are there any prerequisites for your choice?...for example, are you asking what character would be the easiest transaction from Marth? What character compensates for Marth's weaknesses?
If not, expect people to respond based on biased personal reasons...for example, I'd recommend Kirby but that's only because I main Kirby...same thing with "Kiing"
choose your secondary as you chose your primary...through feeling...
If the character's moveset feels compatible with your style of play, then go for it...nothing else should matter.