The following is my opinion only (of course).
First off, it should be light so as to not overwhelm the balance of the rifle.
Leupold Mark 6 3-18 (On the KAC site, this is how the rifle in the picture is configured) fits the bill if you need the magnification. Light and compact, very good glass.
Personally, I see the .223/5.56 NATO round as being effective to maybe 400 yards, so high magnification is unnecessary.
I'm on a 1-6 bender these days. At 1x they work like a CQB optic and at 6x have enough magnification (for me) for use out to 400 yards. It's not the right optic for punching paper, though.
Had my hands on a few and so far I like the Bushnell SMRS 1-6.5 in SFP and the SWFA 1-6 (FFP, the only way it comes). The glass in these two are the same to my eyes, which is very good. The same glass in a high magnification scope might reveal its limitations, but in a 1-6, it is very good.
Both have nice (quasi-daylight bright illuminated) CQB reticles for close quarters. I don't see the need for brighter illumination; if it's light enough to negate the illumination, then the reticles are highly visible already. Both work as well using BAC (Bindon Aiming Concept, both eyes open) as my Aimpoints, with the Bushnell tube almost disappearing at the right eye relief.
The Bushnell is the lighter of the 2 at 18 ounces. The SWFA is 4 ounces heaver at 22. I haven't tried a Vortex 1-6 yet, but they are heavier still (25 ounces) and nearly a half pound more than the Bushy. Still, I've been tempted to try one. There's always the Swarovskis if you require alpha glass, but I doubt it yields much improvement at such low magnifications. For my middle class needs, I think the Bushnell and SWFA fit the need quite well.