Someone above mentioned the following Master System games:
"Quartet, transbot, my hero, slapshot"
I can't speak for slapshot - I never played it. But "My Hero" is the kind of frustrating game that is really good in every aspect except one huge one - in this case, that it has bosses that are impossible unless you learn the exact right move to defeat them. And unless you cheat, this takes HOURS. Other than this, it's actually very good - graphics, sound, collision detection, controls etc are all excellent - but I just hate boss fights that are impossible until you find the one technique that works, then are ridiculously easy afterwards.
"Transbot" is one of the more boring side-scrolling shoot-em-ups I've played - I'm sure its colour scheme (brown on brown) was the inspiration for the original "Quake"'s scenery. It looks boring, thanks to it inexplicably taking place underground; it sort of plays like the awesome "R-Type" but without any of its enemies, bosses (R-Type had some of the best boss fights on the Master System), or playability. There's just nothing fun about it - it's an extremely dull game.
"Quartet" - from what I can remember this was just another annoying platformer with slightly dodgy collision detection. Other than that it wasn't particularly bad, or particularly good. It didn't do any justice to the Master System's technical capabilities, such as they were. If I had to pick a platformer on the SMS that really does justice to it, I'd have to name "Mickey Mouse: Castle of Illusion", which is about a thousand times better than the Megadrive/Genesis game of the same name. Oh, and "Wonderboy 3" is awesome, an all-time classic, but everybody who's ever played a Master System game has probably played that one so it doesn't really count. (I'd give major props to anybody who's actually FINISHED that one without using the cheat code though. It took me months. It's worth it though, it also has the best end sequence I've seen on the Master System.)
I don't doubt that there are hidden classics on the Master System that very few people have ever played, but "Transbot" and "Quartet" aren't them IMO. There was a very good reason why "R-Type", "Castle of Illusion" and "Wonderboy 3" were so popular, and it's the same reason why nobody was telling their friends, "You have to get Transbot! It's such an awesome game!" Some games were just not good enough to rise out of obscurity.