Well if you can't tell by my avatar, it's the last one that came out before Y. I started with the much earlier ones myself in this order: IV, R, D, OG1,2, J, A, W, Masou Kishin, Endless Frontier, the VTX trilogy, OGs, and 30. And out of them, I've beaten, D, OG1&2, J, A, Endless Frontier 1&2, and the VTX trilogy. I also have the side games of Eiyu Senki, Compati Hero, Battle Soccer and even SRW Pinball. I even played the SD Gundam G Generation Cross Rays but crapped out soon as I test ran it.
I'm actually enjoying playing through V, X and 30 as I'm doing all three in tandem before Y comes out all on my YouTube channel. I don't have a best choice per se but 30 stands out for having the strongest message any of the series ever had and it's a motto we all should live by.
Not to mention, getting a metric ton of units from other units not many forget actual exists from other games and franchises.
And that motto is this: