I now own a Master System - suggest some games!

[Centrale]

The U.S. version of Sonic 1 is quite rare, but if you don't mind getting a PAL version (there's no regional lockout between NTSC and PAL) it's one of the most common games. And like I mentioned before, the SMS Sonic games have their own level design so they're worth checking out in their own right (as opposed to just being downgraded ports of the Genesis/MD versions).

In general, definitely check out the SMS PAL library, as about twice as many games came out in PAL territories, with lots of third party support, primarily from PC game developers.

[MrTechnoSqueek]

Wonderboy 3 The Dragon's Trap, I would say is the best game there is for the master system.
Also Deep Duck Trouble Starring Donald Duck, we got it here in the west only on the game gear but you can get a European version off ebay. It is a favorite of mine.Like many others have already said if you find Ninja Gaiden for a good price PICK IT UP! and one last game totally get Golvellius Valley Of Doom

[CrazyT]

nice to see even more great looking box arts. Thought those were sparse exceptions

I also want to recommend gain ground for sms. Some games for me just felt better on sms for some strange reason and gain ground was one of those for me

A lot more fun in co op tho

[George]

Two guns, bro try 4:

[ROJM]

Get KENSEIDEN the original Onimusha..it even has a similar plot to it..wonder where capcom got the idea from...
Almost all the Taito games. If EA was a important third party to Sega on Megadrive and Capcom was to the Saturn..than Taito definatly was to the Master System. Its the only close to arcade perfect versions to games like Chase HQ and Operation Wolf during that period.
Also while the game is a bit bad SHADOW DANCER is also a game i would recommend only because the master system version is the actual arcade version of the game unlike the megadrive game which wasn't.