Of all the combiners I can remember people liking both as a kid and an adult, I think it's Menasor that people seem to like most. I'm not altogether sure why (it might be the fact that they're Decepticon sports cars, and that as a concept alone is pretty cool), but people really seem to have a thing for Menasor. And Motormaster in particular. I remember when Botcon did their Transformers Animated-inspired Stunticons, a lot of folks were VERY excited to see the reveal of Motormaster from that set.
But I digress. In 1986, Motormaster was the coolest of the combiner commanders, I think it's fair to say. He transformed into what is feasibly an evil version of Optimus Prime (Nemesis Prime, anyone?), and had really cool colours of grey, purple and black. And he HAD A SWORD. This was a novelty at the time, and was really a great gimmick, to me at least/ It made him look at least 40% cooler, and was chromed as well, so he looked like a Decepticon pimp.
He was a bit of a badass in the cartoon too, always going after Prime and wanting to wreck him. In the comics, he fared a bit worse, blending into the back ground after the initial rush of excitement of the combiner teams. It always seemed to me that Bruticus was the Decepticon poster child for the Special Teams, after Devastator (who wasn't included in that group). Menasor, being portrayed as critically insane, never got much of a look in. One thing was always consistent though; Motormaster wanted Optimus gone, and as someone who even to this day doesn't care for Optimus too much, this was always a good thing.
I never owned the Motormaster toy, but I went to school with someone who did, so I can tell you that it's one of the good ones. The Scramble City combiners were always solid, but Motormaster had a lot of play value as well, being a badass truck, and really well put together robot. The colours were good, the weapons even more so, and even his articulation wasn't too bad, mainly due to the fact that as a combiner he needed it. Base mode wasn't something I can ever remember being used much, but when was it ever?
Now that Fansproject are releasing the Stunticons set they've produced, I foresee a lot of interest in the G1 versions, so basically, get them while they're hot. As far as I can tell the Stunticons have always commanded a pretty penny on the secondary market, and thats not going to change anytime soon. I will say that I think it's worth trying to pick up a Motormaster at sone point, as it always was a solid toy and a good example of Scramble City tech at it's best.