Ultradot MatchDot (variable dots of 2, 4, 6, and 8 MOA) on both my 10" .22 and 10" .223.
I like the 2MOA dot, but I find that I'm using the 4MOA dot and larger more often than I thought I would.
What I do is to pick a dot size that fills in an inner circle of the target (depending on distance) and have it sit perfectly centered in that circle.
While you don't actually see the bullseye that way, it's really a very accurate way to get your aim dead-center on target.
Like a lot of people, I always thought that the smallest dot size was best, but I'm quickly rethinking that theory.
As far as the Matchdot goes, I've personally found that it's a great scope that is built better than it's under $200 cost would suggest.
I have an AimPoint CompM3 on one of my AR-15's, but am thinking of spending a bit less and putting an Ultradot on my other AR-15.