What do you want the dot to do?
Some applications prefer a smaller dot (2 or so MOA for precision) others like larger (8 MOA for faster acquisition)
There are options for Red or Green. I find the green dot easier to see.
Holosun offers models which have 2 MOA dot, 32 MOA circle or both. These are available in red or green.
Small and light or do you want a big window (like the SRO)?
More information will lead to better answers.
(Otherwise I will just say get a Holosun 507c Green since my preference is obviously universal
)