There are four:
Number one must be my first TF, before I even knew what a Transformer was. My uncle bought me Optimus Prime. Ironically he got it for me because he knew I liked toy cars and trucks. He didn't even realize it transformed into a robot, and was quite shocked when I took it out of the box, rolled it around for a bit and then began turning it into a robot. Initially he thought I had just broken the toy that he got me for my birthday, only to realize, as I just had, that there was more to this 'truck' than meets the eye
. I was hooked from then on and Transformers became my favorite thing.
The second (and only slightly second) best would be Powermaster Optimus Prime. The bulk, semi-cartoon accurate cab/robot mode, the powermaster gimmick and the awesome combined mode was just too awesome. And those massive guns and shoulder cannons made it all the better. I loved (and still love) that figure.
The third is the trusty DoubleSpy, AKA Punch/Counterpunch (G1). What an awesome and versatile toy. I'd spend hours just transforming it over and over again between its 3 modes and often gave Punch 4 arms by folding out Counterpunch's arms underneath.
Finally, my fourth would have to be a tie between Sixshot and Doubledealer. Sixshot for obvious reasons. Marvelous bot mode and years of play value in a 6 mode figure. Doubledealer was great because I saw in him much of the appeal that Punch/Counterpunch had, plus the Powermaster gimmick, which I always thought was cool, especially since he came with 2 of them, one for Autobot mode and one for Decepticon mode.
I had Scorponok (G1) as a kid as well as Metroplex, Ultra Magnus (the good one with the painted face/antennae etc., not the solid blue KO looking one) and a slew of other figures, but they never gave me the joy that the above 4 figures did.