HA! I'm not alone on this! Yeah, he looks absolutely stunning, and it helps that he's my favorite IDW character
I voted First Aid on account of loving the character and liking how the toy resembles his MTMTE self, but overall I'm tied between him, Streetwise, and Cyclonus. All three of them are just SO well done that you can hardly tell they're not new moulds, and they do a great job of looking distinct from the originals.
Can't really count Motormaster, IMO, as I staunchly believe Optimus was a tweaked version of the mold released first so that the Stunticons would time closer together. So, I'm opting with Slingshot.
I voted Thundercracker. I just like his look. Although there are things that could have been better. Also he's too large. I don't think that Cyclonus is good. He is in every way inferior to the deluxe version.
First Aid seems to be getting a lot of love. That's nice, considering his sympathetic portrayal in The Ultimate Weapon.
Viper is such a neat idea. I'm surprised that one hasn't gotten more votes! That's my pick. I also love Motormaster but that's more of a "premold" with Prime feeling like an afterthought. ^ Agreed.