Cheap brands and optics will have laughable parallax and performance issues, especially the eotechs and other fancy scope types.
If you shell out a bit for a vortex crossfire red dot, you'll never need anything else. I have this and a sparc ar and they are both top notch.
That being said, I recently went for a vector optics maverick gen 2 which was about £60 on sale at opticswarehouse and my god this thing is quite amazing as well. The parallax effect is pretty much unnoticable when shifting my head, and I've used it for several games days, even on my sniper rifle for fun, and it has held up very well. Would recommend that specific sight, can't vouch for the others.