I have an Aimpoint PRO on my 5.56mm AR, and a Comp M4s on my D/I .45. On AR15.com, you can find used Comp M4s for a little over the price of the PRO, new. If you watch, you'll find awesome deals.
To me, I stick "best red dot" to Aimpoint. Yes, they are a little more pricey, but you rarely see them fail. The PRO is probably the best value in their line, especially with the mount being included. I like the Comp M4 series, as they use AA batteries. The PRO batteries are harder to get, but when you replace them yearly... it isn't THAT bad.
I actually did pick up one of those SIG branded Holosun sights for my father's Colt (going to be a Christmas present, and found a great deal). Opted for the AA version, as it would be easier for him. I've heard good things, however, I don't put them on par with Aimpoint.
What it really comes down to, is how much "best" you want to pay for.