So you're basically asking which mold is most suited for each Optimus incarnation? That's a bit hard, but I'll see what I can recommend.
Generation 1, Generation 2You can never go wrong with the G1 original. One tip I should give though is to look for Japanese reissues if you have the money, as the US reissues have the smokestacks trimmed and the missiles elongated.
Another version worth seeking out, aside from the desired Classics Voyager, is the G2 Laser Optimus Prime, simply the best toy of his time. He even has a new trailer.
Beast WarsThere are no more than 3 Optimusses (well, including the Japanese Convoys) in this era, one for each series. Since you don't want animals in your setup will casually skip that.
Robots in DisguiseYou already have the only large toy, and I don't blame you
Unicron TrilogyThis one's iffy, not only because there are indeed three separate designs of him, but Japan first considered their version of Energon and Cybertron separate series and not sequels. So Energon and Cybertron Prime were separate characters at first. For the sake of simplicity, stick with the Cybertron version.
Live Action MoviesThe general opinion is that RotF Leader Prime is the best out there, and for good reason. Since you already have one and I assume want a second, look at some of the (really expensive) Japanese versions. I'd recommend looking at either Buster or Striker, or if you have a lot of cash to blow, at Jetwing.
AnimatedEasy: Voyager wins hands down. If you want a special display, look into the shiny Japanese edition.
PrimeUnlike Hasbro, I do
not consider WFC and Prime Prime the same character. As such, I prefer the First Edition for lack of obstructing gimmick. Not to worry though, rumor has it that Hasbro is looking into a re-release in Fall.
War for CybertronAnother easy one: Generations with the design from the first game.
Top 5 Priorities:
1) BW Neo X-7 Elphaorpha
2) BW II VS-18 Powerhug vs. Autocrusher
3) Universe 2003 Night Slash Cheetor
4) RiD Landfill (regular, not the yellow)
5) BW Convobat (red) & Megalligator (purple)
Also looking for any clear Arms Microns, like Shadow (Gora & R.A.), Hellflame and Shining (O.P. and R.A.).
Preparing to move to the US!- JelZe GoldRabbit =:3