Well, my choice in Wars and World was the same, and likely will continue to be in OverWorld.
"Omae o korosu..." *fires Twin Buster Rifle*
Now don't get me wrong. I love how much damage can be done with the likes of ZZ, Nu & Hi-Nu, Unicorn, God Gundam, Double-X, Turn-A, and 00 Raiser. But there's just something about the units from Wing and Endless Waltz. Maybe it's nostalgia since that was the first series I watched. But you can't deny that they can kick ass with the best of 'em. (Now if only the Twin Buster Rifle's main shot wasn't outdone by the twin-firing, and if Heavyarms got all its missiles back...)
Besides, with a little patience and luck, you can assemble quite a few of those other Gundams. Depends what can be extracted from Tornado, Phoenix Zero, and Phoenix Gundam and where the available mooks' evolutionary lines can lead (if I'm not mistaken, you could eventually turn a freakin' Zaku II into Zeta Gundam in World, but a lot of steps had to be taken along the way)