My favourite Primes,

G1 (can't really beat the original, except with MP-04 and MP-10)

MP-04 it's a glorious toy, solid, great transformation, looks spectacular.
MP-10 is almost as good as MP-04, it's cleaner, but doesn't have quite the same appeal in the Robot mode.

I quite like the animated prime, I realise you're not a fan but the transformation of the earth mode voyager is not overly complex but is interesting with the spinning gimmick.
The Cybertron mode deluxe is a pretty cool take on the character.
both robot modes are well articulated and can be posed quite emotively.

First Edition TFP Prime is a brilliant toy, well engineered and articulated. (be sure to avoid the one with brown doors which was a re-release) the first release is pretty much impossible to pick in-pack and potentially a lot more expensive if you can find it.

the Generations voyager prime is a good solid mould, it's starting to age a bit in my opinion but is still an excellent modernisation of the original character.

War for Cybertron Prime is a good mould, complex but fun.
FOC prime is small and simple but does look like a fist on wheels as described by the game designers.