Honestly I'd have recommended Monk because even though it has 2 combos, that's almost all it has aside from a few oGCDs and buffs.

But if you want the simplest class, I agree with Rongway that Dancer is probably your best bet. It has 4 primary buttons, a 12 combo and then 2 other buttons that have a 50% chance of proc'ing off of 1 and 2 (consider them 3 and 4). And 3 and 4 have a 50% chance of proc'ing a feather that you use between attacks for more damage.

It does have other buttons (and to be honest, all the classes have more than a few buttons that get used), but it's still considered the simplest class in the game. And it's basic gameplay is set in stone by level 50.