For myself, my favorite optic of all time would have to be the Cronen Mini Pro (MWII). It's a simple, yet elegant sight with a thin frame and rounded top. This was also the same optic that was used in the original MW2 and it still holds up to this day. In fact, the Cronen optics generally are really good, especially the mini-reflex in MW19.
There's also the optics with the floating dots, like the ELO from BO3 and variations of it later. I really like those ones because of the clearer sight picture they produce.
Oh yeah, the simple blue dot is my preferred reticle choice.