Interesting how there's a few varied answers here. Doubt anyone will say "512k" or "128k" but so far we've got beige Plus, SE, Classic and Classic II already.

Personally, I think the Classic is the most common. Especially the educational one without HDD. I think I must have about 10 of them 8-o

Also it depends on what this "mod" is but the Classic is probably the most recognisable to most people too.