My personal favourite was WonderBoy III: The Dragon's Trap. The characters are those classic, big eyed sprites that makes me immediately think "Master System". It was a pretty cool game too. You could turn into different animals which behaved in different ways. In a sense, it was a metroidvania as you'd need to unlock certain characters to reach new places. But yeah, definitely one of my biggest inspirations.
Alex Kidd in Miracle World had its charm too. It looks simple, but it was brutally difficult. It actually came baked into my Master System and would run whenever I started it up with no cartridge.