I have a fondness for Master System because it was my first ever console. I had a model 2 system with Alex Kidd in Miracle World built in, plus Sonic the Hedgehog and Aztec Adventure cartridges.
I moved onto Mega Drive quite quikcly after, but when I started actively collecting retro stuff, the Master System was a system I targeted early on. This was back when you could get amazing deals on eBay and I picked up the console plus 9 or 10 games for about £15-20.
Off the top of my head, these were the games. I kept most of them:
Phantasy Star
Special Criminal Investigation
Marble Madness
Castle of Illusion
The Lucky Dime Caper
Asterix and the Secret Mission
Fantasy Zone
The New Zealand Story
Sonic the Hedgehog
Sonic the Hedgehog 2
I got some other bargains for Master System on eBay too. Golden Axe Warrior for £12, Ys for £15 and way back when Wonder Boy III for a princely 5p. Although it's not so affordable now, Master System is still a fairly cheap console to collect for over here in Europe.