The type is up to you, whether it be red-dot, EOtech, ACog, reflex, or whatever. It's all based on what YOU like and what YOU'RE comfortable with that will compliment YOUR play style.
That being said, you're best off to roam around the internet looking at optics and/or going to local airsoft shops (or gun stores) and looking through a few optics to get a feel of the differences between them.
When doing this, consider your ACTUAL effective engagement distance of your gun and how you plan on using it. That is to say, if you're playing CQB you may not want any zoom.

That being said, what's your price range?

-mobile device-

 

I've run Eotech, Aimpoint,, T1 style, 30mm tube style, Open Reflex style, and even a 4X scope on my old P90.

Look at some pictures and see what appeals to you:

https://www.google.com/search?q=p90...sch&source=lnms&sa=X&ei=sMhCU7LaMe6uyAGc9oGYBg&ved=0CAYQ_AUoAQ&biw=1109&bih=567

 

One of the smaller eoTechs or smaller dot sights


-FS

 

Get an ncstar reflex style sight


• Clips are for papers, Magazines are for guns.

• 12g bb's don't suck! I almost get hits from four feet away!

 

I would get an AMP micro red dot sight. Its really accurate and always stays sighted in. On the front lens it comes with a mesh light protector. This acts as a shield for the lens. You can find it on airsoft GI's website.


Stay softing