Megatron, Starscream, Shockwave, Soundwave, Jazz
Why? Not because of any particular thing about them that I can put my finger on and say, "these are my favorites for this reason," but because they have a certain je ne sais quoi about them that appeals to me personally - an intangible quality that makes them distinctive or attractive.
First in their fiction, then as toys as well. See above.
I wouldn't say any were disappointing, other than to be under-used. Too many characters with not enough screen time - one of G1's main let downs. Jazz, Prowl, Ratchet, Bluestreak, Sideswipe, Sunstreaker, Elita-One, Hound, Cliffjumper... to name just a few Autobots, and there were also many Decepticons I'm sure, including Shockwave.
For me, definitely Jazz and Shockwave.