Expensive but dim - Mepro OPMOD M21 Red Dot Sights Reflex, HC Reticle

Unless you are constant exposed to outdoor and well-lit areas (like in the desert where this is invented), you will find the dot very hard to see due to poor ambient lighting. Especially in in-door ranges.
Also, the blue coating filters out red spectrum, hence most red markings on the target papers are invisible.
This is a quality product, but the large MOA dot and the difficulty in indoor environments make it not the best to mount it on a mid-long-range rifle.
I added the side-illuminator, even at max setting it is still quite dim.