tbh I don't even know lol.
If it wouldn't be a logistical nightmare, I'd love to have people play that silly touhou doujin game I played (Meirin in fists of fire).
But a more practical answer, I'm not sure. I suppose whatever it is I should be familiar with? As in, know some of the meta so I can better run the tournament?
But then that would leave only, like, Monster Rancher and Pokemon lol.

The magical drop tournament was cool though...
... Could we play Pop'n Pop? That one has Sayo-chan in it...
Or maybe Puchi Carat? (at this point I'm just naming taito games)

I suppose the game itself doesn't matter as long as it's something fun to do with a lot of people!

... sorry maybe that's not an actual answer.