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Nice choice for your first AR!
My favorite RDS is the Aimpoint T-2, best deal at LaRue as a package deal. It is hard to justify the price so my second favorite is the T-1, but if you don't need NV the H-1 is just as good. I've been very happy with my T-1s. I also like the Aimpoint PRO, but I'm not crazy about the mount it comes with and the less available batteries it uses.
If you are going variable, you will need to figure out what your budget is, but I like the Trijicon TR24R which is very good for fast shooting out to 300 yards. If you want a more precision reticle you might want to shop around, but I'd definitely go with a 1-4X or 1-6X.
Some folks really like the Vortex PST 1-4X, I've never used one. Some like the Leupold VXR Patrol 1.25-4X or the SWFA 1-4X or 1-6X. I have a SWFA 1-4X and it has great glass, but is a bit on the heavy side. Some like the SWFA 1-6X. I think these are made in Japan which is FAR better than the Chinese made optics like the Primary Arms 1-6X or the Vortex Strike Eagle 1-6X.
Personally, I would get an Aimpoint first, then pick up a cheaper variable later for playing at the range.