The factory rail is not reliable, it's a poorly designed system that doesn't have a point of reference when it's installed. Quite often it will cause the optic to cause your impacts to be too high, sometimes too low, it just depends on how far toward the receiver the rail is clamped. There are only a few rails that will work properly, most either don't fit well or wont properly align the optics to the bore. M14ca and Ultimak are my favorites, I have the Ultimak but if I had to do things over again I'd probably buy the M14ca Scout Handguard.
As for the optics themselves, I've tried several;
Nikon Force XR
EOTech XPS2
Aimpoint Micro T1
Leupold Scout
Vortex Red Dot
Bushnell TRS-25
The best red dot was the Aimpoint Micro (T1 or H1) but the Bushnell is an inexpensive compromise. The EOTech is good if you need a tactical optic but I think it's just too much for the average shooter - too big, too expensive (plus EOTech admitted that there is a parallax issue when the temperatures change). The Leupold was nice but I went with the Nikon because the rifle is accurate enough that the extra magnification was usable, I've can keep my hits down to 2 MOA out to 400 yards.