Some of the following may already be known to you so don't feel I am talking down to you since I don't know what your background experience level is.
First off, for the most part magnified optics will not allow you to cowitness the sights through the optic. If you want to be able to do so, you are looking at a riflescope that offers a true 1X mag setting, a red dot sight, or a non-magnified prism scope.
Whether you choose to have an exact co-witness or a lower 1/3 co-witness is a matter of preference. Many red dot sights and other optics that are intended for an AR platform will either come with a riser built in, or an add-on riser. Most of these will provide for either an exact co-witness or a lower 1/3 co-witness. A few models will come with two risers to provide for either.
In case you don't know, with a red dot sight that is set up for an exact co-witness, the red dot when centered within the optic by adjusting your head position will pretty much exactly align with the tip of the front sight post when aligned within the rear sight aperture. With a lower 1/3 co-witness you can still see and use the back-up sights in the lower part of the optics field of view but the red dot, when centered, will be seen well above the tip of the front sight post.
On a standard flat-top AR receiver you need to mount the center of the optic about 36-37 mm above the top of the Picatinny rail for an exact co-witness. For a lower 1/3 co-witness the center of the optic will need to be mounted a bit higher, around 40 mm or so.
If you want an exact co-witness with your back-up sights an option to consider is the SIG Romeo 5. This has an instant-on feature that preserves battery life and allows the illumination to come back on at the last intensity setting. It has a crisp 2 MOA dot and comes with a 1.41" high riser that is removable. I have purchased two on sale for just under $120 and have occasionally seen it listed even cheaper. Sportsman's Outdoor Superstore currently has it listed at $119.99.
There are a number of red dot sights made by Holosun which are very similar to the Romeo 5 that also have the shake-awake feature.
Another optic I have used is the Vortex Spitfire 1X which is a non-magnified prism scope. Prism scopes are good options if you have astigmatism which can result in smearing and distortion of a red dot sight. The Spitfire has an etched reticle so you can see it (in black) even when it is not illuminated. The reticle can be illuminated either red or green and the reticle's 3 MOA dot is surrounded by two concentric aiming circles that allows for rapid target acquisition. This optic comes with a removable riser that mounts its center 40.4 mm above the rail allowing for a lower 1/3 co-witness.
Another Vortex optic I have not used, but which seems to be quite popular, is the Sparc. This is a non-magnified RDS with a simple 2 MOA dot reticle. It does not have the shake-awake feature but it does come with two different risers providing for a mounting height of either 37 mm (exact co-witness) or 40 mm (lower 1/3 co-witness).
