Title says it all. I'm not gonna get to everything but I'll try. I left out Primal to ensure I can cover as many bases as possible on Prime. I really wanted to include Powermaster Prime but he's similar to G1 and there's limited spaces. Same reason I didn't include the Marmon from his AOE Evasion Mode. I didn't include Fire Convoy's alt mode because even though that's technically Optimus Prime domestically the character wasn't originally intended that way and I figured it's more prudent to include alt modes designed with Optimus Prime in mind. My vote goes to the Western Star. Sexy.
I liked him much better as a truck than as a truck. altough he did look good as a truck, but not as good as when he was a truck.
RID 2001 was my favourite. The G1 cabover is an icon, but a fire truck suits Optimus's character so well. Though there have been several generations where he turns into one, RID was definitely my favourite. The Cybertron fire truck is a close second, but it just felt a bit too cartoony to me, which is almost entirely due to the third 'Spaceball' mode.
The tanker is fun, but I really like the "lol look at me" aesthetic of the 2007 Peterbuilt. "Do you think I should have flames on my car?" Yes, Optimus. Yes I do.
I voted Cybertron because I grew up (and still own) that toy and come on its a truck with giant rail gun thingos on it. But if I had a second vote it'd be the '84 alt mode.
1984 Cab-over is iconic and I'm sure we all love that design but my favorite has got to be the one from TF Cybertron. A fire truck fits Optimus's character better imo add to that the leader class toy is one of the best Optimus toy of all time (also imo).
