Fuck, that's a really hard question.

my brain is telling me 00 but I know that's wrong
G is a very good show but not very Gundam if you know what I mean, and it has some dumb plotting/pacing issues
I liked X a lot but my brain is saying that it definitely isn't the best
fuck SEED
Wing is lulzy but also not a great show
part of me is thinking Zeta but again, probably wrong
g-reco is REALLY pretty but the pacing is a total mess
witch from mercury is alright but I cannot in good faith say it's the best

I really don't know!

...actually fuck it, Build Fighters. ヽ(´∇`)ノ
I'm kidding, but it was a lot of fun.

maybe Turn-A is the real answer, but I haven't seen it in ages so I can't say that for sure
it's the name my brain is telling me is the least likely to not be the #1 spot ┐(ﾟ～ﾟ)┌