Clayfighter was on the NES and Genesis, it's on the Virtual Console, and it also had 2 sequels on the N64.
We've already done a ton of games that don't have widespread ports - probably half of the series or more is of games with two ports or less:
Snatcher
Nights
Toe Jam and Earl
Punch Out
Xenogears
Kid Icarus
Ristar
Solar Jetman
RC Pro Am
Diablo II
Clayfighter has more ports than any of these, I believe. I know we've brought it up before in conversation, too (Ack supported it, for one).
Besides, shouldn't we base the rest of the 2010 calendar on the list people may have been purchasing from already:
Later:
MUSHA - Genesis/Megadrive, Wii
MULE - AtariPC, NES, PC
Spy Hunter - Arcade, NES, Arcade Collections
Those all seem like good picks...