So I thought that CM- er... the GSZ author had the best article, but not the best suggestion(s).
I think the best suggestions are metamorph, GSZ, scuttlemutt and tinker. All of them clearly require some designing the cube around, but, you know, that's fine. I do disagree on power level ground with some of the discussion, especially with tinker, which seems sweet if you can build a cube that value artifacts rather than using them as haymakers and finishers 5 turns early. My personal feeling is that metamorph means you can do ugly stuff in colours that shouldn't be able to, but I respect the opinion of the author that, you know, that's the point; and with that as your cube premise, seems like it would fit in nicely in multiples (possibly even in the same cube as multi-tinker). I'm not sold on multi-GSZ; I know you can construct an environment that supports it, and it seems like it could be interesting, but the sort of environment where you draft whatever green you feel like, and then jam in every GSZ you see so you can't have a bad draw doesn't seem like my thing (as I have a low tolerance for power). I could see going to a one-of to enable the odd interesting deck (which I realise that multiples is the point of), but the incentive is in the green creatures, it just so happens that GSZ short circuits the deck building to some extent, which seems awkward to me. Your mileage certainly will vary with that one.
So I voted scuttlemutt, as I'm sure it's something people won't have considered, and really does seem to add an interesting dynamic if you have multiples, although again you need to design your cube around it a bit. Even then though, it's a 2/2, its fixing, its ramp (assuming your cube is slow enough to ramp with a 3 drop), and everyone can use it. With a bit of playing around, it might be interesting to include like 4 of them, and play a bit more with protection from $single_color as then there are opportunities to work around that in a mono deck. Its certainly the one I can see getting the most mileage out of personally.