Honestly it depends on the look you like. I don't actually use my optics. I opt for the clean and minimalist look of irons (I'm a big fan of the KAC BUIS sight plus triangle front post sight on the M4) also in airsoft you're not really going to use anything higher than 1.5x unless you're scouting around just because we don't get that kind of range on our guns. Other than that my preference is the Aimpoint (Comp M3) but I think the big thing these days is the Aimpoint T1 since it's super high speed tacticool.
If you like the T1 take a look at the Primary Arms Microdot. They're pretty high quality and actually work on real steel and hold their zero (don't use airsoft optics on anything larger than a .22LR because it won't hold it's zero).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE