Buster Prime, or the ROTF Optimus mold.
IMO, it seriously must have taken alot of work to get it that accurate to the movie in both modes. Plus, buster has great paint, down to the pinstriping on the flames. I cant be 100% sure, but it could very well be the TF with the most paint apps. The whole body is silver paint!
HA Jazz is also slathered in silver paint.
For me, based on my collection,
Leader- Rotf Optimus Prime for all of the Previously mentioned reasons
//honorable mention to Beast Machines Ultra Jetstorm, he's great on both modes, his geared gimmickry is superb (and awesome!) and he just looks really good overall, if not totally screen accurate.
Voyager- this one is tougher, but I think I have to give HA Jazz the edge over the original Optimus Primal. Primal is objectively pretty great, but a lot of love(and paint) went into Jazz's design, and his joints are top notch.
Deluxe- tie between Generations Drift and Warpath. They're both awesome in both modes, and are just designed really nicely. Drift has swords, swords man! And Warpath was one of my favorite toys when I was a kid, it's such a great update of the character.
Scout- Easily, Rotf Breacher. I love his modes, transformation, and I immediately fell in love with his head sculpt. His only real flaw are the chunks of arm kibble, but that's completely ignorable.
Legends- Megatron loses points for being difficult to transform, Cybertron Jetfire is pretty cool, but his articulation leaves you wanting a bit more... I'd have to give it to Prime cyber verse Cliffjumper. I really like this mold, it's so well articulated for the size class, and it just looks good.