Tough choice, just 3.
For me, a perfect Transformers is great looking and fun to play with. It dosent need to be expensive or exclusive to be special.
My first choice is a no brainer. If you know me, you already guessed: God Ginrai from the G1 Japanese series Masterforce.
He's huge and there's so much you can do with this set! Sure, he's a brick and a partformers, but he's my #1 for ever and ever. Even if it's just a reissue. I could apply the stickers myself and that was a great joy in itself.

My second choice is Masterpiece Starscream, G1 color and the Ghost version. I do own MP-01 (last shot) Optimus Prime, but his vehicle mode is pure fail. I also own MP Rodimus, but he's fristrating. However, MP Starscream is hot in all modes and quite fun to transform. He also feel sturdy in my hands and I never fear to break him. The hip kibble fault is easely forgiven by just unplugging them and plugging them on the legs. I could buy the MP-11 retool of Starscream but 150$ + shipping is an obscene ammount to pay for a minor mod. Anyway, I love MP Screamers enough to have spendt more than 100$ to display them properly.

My 3rd may surprise you.
It's Marvel Crossover Warmachine, car version. Before him, it was Ironman Car version. However, the new Warmachine have a far more superior paintjob plus added glow-in-the-dark gimmicks! He's of a decent size, have a fun transformation, and is very awesome looking overall. He's a lot of fun to fiddle with! I bought one to open and another to keep sealed because of a buy-one-get-the-other-at-50%-off promo. The funny thing is there was a mistake at the register and Québec laws tell that in case of a screw-up on the price at the register, you have 10$ off the supposed lower price (if the thing ios worth 10$ or less, it's free). So, because of that, my first Warmachine cost me 23$ Total, the second one, just 2$! Anyway, even at full Pprice, Marvel Crossover Warmachine is perfect in every ways.

If I could bring in a 4th and a 5th, I'll chose HftD Leader Starscream and Animated Voyager Bulkhead. They are both great to look at and great to play with.