Alright. After ranting, raving, arguing, and agreeing (also straying) all over this thread, I'd like to officially cast my nomination.
"Best" is a tall order, but this design is certainly MY favorite. And I'll take it from any era.
When you say Transformers, the first image that pops into my head is this guy.
Why Silverstreak? Why not Prowl? I don't know. It's just always been that way since I was 9 years old. Any of the 3 characters are fine really. I personally just like this one the best. When I was 9, the Datsun Z28 was my favorite sports car. Then this great toy came along and it was the car but, it was also a robot?!? My other favorite thing!
I said in another post, that the design esthetic I find most attractive, is what I would call, the Takara style. Transformation or engineering design has and will always be an evolving design process. It should be. We've had 30 years of innovation, and it's what keeps these old characters interesting. But, for Takara the aspect of character design has virtually been a constant. we see it most in their head sculpts. Silverstreaks head has always been my favorite. As a 9 year old I always assumed the autobot logo was HIS head.
Design wise, what makes TFs different and unique from other robot designs? Dare I say it? Kibble. A Transformer should look like a car, or some other vehicle, unfolded and stood up. That's what they are. If that's not what your into, then maybe Gundam is for you. I like my TFs to have car parts hanging off of them. For me Silverstreak is the perfect poster boy for this idea.
From his car door wings down to his tail light toes. The way his awesome head pokes up out of the hood of that sexy roadster. The way the the front end creates that iconic and heroic chest. The way the front wheels sit behind his shoulders. And those legs! My favorite Transformers legs ever. He's wearing bell bottoms over his funky platform shoes. DISCO BABY! I mean, technically he's from the disco era, Right? So am I. Oh and just to reinforce that idea, he's got roller skates, if he so chooses to employ them. From G1 to MP his appendages remain almost completely kibble free. Allowing for a wide range of motion.(let's just ignore the fixed leg situation on G1.)
Which brings us to the engineering of Streak. The way his arms fold under the hood is the best way of dealing with arms on a car. Plus you can do the funny, car with arms mode. They so perfectly deliver those shoulder wheels to their precise location. That method hasn't changed from Streak to Streak, and it's been used by so many others. Heck! I think Streak invented the 180 waist swivel. Oh and his legs again. A great example of, "Yer never gonna make that work. Oh? You did it!". And the windshield folds right out of the way, safe and sound on his back. Optimus your great, but I bet Ratchet is sick of replacing those windshields every time you take a blast to the chest.
If you ask me, Masterpiece Silverstreak is the perfect example of what that line is about. Taking the old design ideas from the original toys and, by using 30 years of engineering innovation, making them better, and what we always hoped they could be. (I don't have Prime yet, but I'm sure he exemplifies this as well.) Not that much has changed for Silverstreak though.
And on a rainy day, with nothing else to do, I'll even settle for this one.
s in any given line, and I like to level the playing field. I also think we are gonna see a lot of re-issues of MP10 which is why I'm not in a huge hurry to scoop him up. 
The G2 versions may be based on Pyro and Clench, the "leaders" at the time those symbols were created.
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