Leaf Green, because I like Venusaur more. ;D
Though seriously, I have (or at least ended up with) both versions, but Leaf Green is the one I prefer since it was "mine" in the first place, it was the version I actually bought for myself. The Fire Red I have was actually my sister's copy at one point, but it ended up in my care whoops. I think I chose Leaf Green in the first place since my first pokemon game ever was Blue, and in place of Blue not being one of the two colors available here ...Leaf Green was the next best choice.