>>58479101
Probably Ash Gren.
Slightly higher BST, more optimized stats, and can hold an item, and you can even put it on a team with other megas. The only downside is that you need to get a KO, but you want to get a KO anyway. It's really not too hard to get one.
Only thing that can make Mega Gren better is if it gets a really good ability.