Wonderboy in Monster Land is probably my favorite non Phantasy Star game for the Master System. It's a side scrolling adventure game, kind of like Zelda 2 and whatnot. You get to play an action adventure game, but you can collect items and money. And with said oney, you can buy new equipment like armor and magic and stuff. I'd recommend it wholeheartedly.
Yeah, I've got Wonderboy 4 (or is it 5) for the Genesis. Cool little game. I like design of the areas and dungeons - nice sense of exploration. Amazingly, Wonderboy in Monster Land actually won't play on a Genesis (with the Power Base Converter) unless you are using a Master System controller (as far as I understand it). Weird.
You should also give Zillion a shot. It's pretty neat, although it's also more of an adventure game. The closest thing I can relate it to is Metroid. You are in a huge maze and you need to input codes into computer terminals to access further parts of the maze. You can level up by collecting Opa Opa (from Fantasy Zone) icons. You can level up your gun and you can recruit two more characters. But it plays more like Metroid than an RPG.
Sounds very cool. I've seen copies of it around quite frequently, also it's sequel (Trizillion is it called?). From what I read about it, it didn't sound like my thing but hearing you describe it inspires me to give it go, which I will.
Truth be told, the games you originally mentioned are about... As RPG as you are going to get on the Master System. But still, they're some decent titles.
I knew the Master System had a pretty weak line up getting into it. Still, you're right, lots of the titles sound interesting.