My three are (In no order):
1) Jazz - yeah, the lack of a locking chest is a bummer, but otherwise a wonderful toy.
2) Warpath - Amazing in both modes, a dream to Transform. Still one i play with the most.
3) Mirage - One of the first and still one of the best. I love the articulation and futuristic
sports car look.

its funny though how so many of my favorites have third party editions. From Reprolabels to add on head and weapons, alot of my favorite guys have work done to them. The three above are all without adition (though i plan to get repros for Jazz and Mirage sometime, they just arent a priority).