I'd say a basic 2-4 MOA red dot...as to what specific one, it would depend on your budget and what the end purpose is.
If you’re just going to be running it as a range toy/plinker, any of the $100-200 budget dots by Sig, Vortex, Bushnell, Primary Arms, etc will do just fine.
If you’re going to want to depend on it as a serious defensive tool? I’d look at Aimpoint, EOTech, or Trijicon.