I just saw a post on one of the forums where a relatively new shooter was looking for suggestions on his first optic. Below is the content of the PM I sent him offering my opinions on the matter. This is the distilled, one-page-or-less, version. Being the wordy SOB I am there may be a more in-depth version to follow.
I tend to see a fair number of sighting systems. not sure what class you're taking, but you might consider shooting it with irons so you can learn the fundamentals and so you can get a chance to see other people's optics before you buy.
But, if you're set on buying optics first, here are my recommendations for what they are worth:
ACOG - TA33G-H
Best balance of offering magnification, eye relief, speed at close range, and light weight. Get, or make sure it comes with, a front flip-up lens cover and run the cover down at close range, with both eyes open, and flip the cap up (open) for distance and consider closing one eye for precision.
Aimpoint (small) - H-1
The item I linked to comes with a mount, which may not be the best mount in the universe but it will get the optic at a good height for the AR and get you shooting. You can worry about upgrading later. Do not bother spending money on the T-1 as all that gets you is night vision settings that will result in more, useless, clicks on the dial for you. Upside here is compact size and light weight, downside is limited head position and higher cost.
Aimpoint (large) - PRO
Larger, and heavier, than the H-1 but less expensive and more forgiving of head position, plus a smaller dot for better precision at distance.
Eotech - XPS 2-0
Good optic for speed, but gives up battery life, ability to be left on at all times, resets brightness every time it shuts off, etc. But it's good for shooting fast at close range if you have a gun and the ability to run it that way. If you don't, IMO the negatives outweigh positives. For me, this is a very limited use optic.
Of the above, I have all of them or close cousins to them, and given the market as of this writing these would be my choices and the applications for which I'd choose them. I think the PRO is the current best all-around, the H-1 the best for "CQB", the TA33 the best outdoors or rifleman's optic, and the XPS the best gamer optic.