I thought it obvious, but I should think ...
the _intersection_ (overlap) of the above listed devices.
Not somethign suitable to just one of them
ie
For first round or two, to keep game picking out of the equation, I'd pick an arcade game that runs well on MaME4ALL, since thats available on pretty much everything. But a little uncommon so its not something everyone has seen (so we can get people to try a 'new' game). And one with a simple scoring system, so we can just say .. "order people into 5 groups by score, top group gets 100 points, each group thereafter gets 20 less; 100/80/40/30/20 points". But really over time, games from NES, Genesis/MD, SNES, TG16/PCEngine, MAME, 2600, C64, Vic20, Spectrum, ST, Amiga, that sort would be fine. Maybe even dosbox, though that rules out most platforms since PSP/etc can't perform well there (though people could use their desktop then, but it might be too much an advantage/disadvantage to use a PC, against mostly mobiles, so I'm sort of leaning towards mobile platform play, but I'm not a dictator.)
Hence, Legendary Wings or something, or Toki, or somesuch.
Then can spend effort setitng up a forum (?), getting names and setting up a tally page somewhere or in the forum (forum post pinned to top maybe, one for each season?)
After a few rounds, can start a poll-thread to pick a game .. show say 4 games from various emulated systems, and see who the vote goes.
If you like, I can pick games for awhile, but the scoring system could be work, but the first few games wil be easy like above, so its ont really a problem.
jeff
edit: ie .. there are a cuple retro gaming leagues around (I just found out), but I brought it up here so it'd be faces we know, witrh platforms we know. If you want a general league, theres the 'retro gaming league'; for Atari ST only, theres "STOT" league. But lets do 'the gp32x and openpandora boards league of retro gamers', if you catch my drift. See, this way I can do some beat down on pickle and its personal