What was the best SMS game?

Considering the nature of what brought us all to this site, this might be a loaded question, but I'll ask it anyway. It's been twenty years since Sega dove into the console market headfirst with their Master System, the main rival of the Nintendo. Which Master System game do you consider to be the best overall title?

Phantasy Star definitly.
Wonder Boy III: The Dragon's Trap was also pretty good.

I'm in total agreement with Lucas - Phantasy Star is the best. If you take it out, the list goes something like:

1) Wonder Boy III: The Dragon's Trap
2) Spellcaster (a real hidden gem for the SMS)
3) Ys
4) Miracle Warriors
5) Alex Kidd in Miracle World
6) Zillion
7) Outrun (darn good port for an 8-bit system)