[Brian McIlmoyle]

The answer is .. it depends.
How much Money you want to spend
How much you want to carry
How your typical CQB game is played
How the others in your fireteam are armed.

The best CQB "suite" in my opinion is.
MP5 , my preference is the "classic" full stock. This weapon is very well designed for CQB, and is a dream to shoot. it is very intuitive to aim and with practice reflexive shooting becomes second nature. it should be fitted with a single point sling that keeps the weapon handy when slung but also allows it to be cleared to the non dominant side when necessary. Don't bother with any sort of RDS.. the iron sights are good for CQB. RDS can make you tunnel visioned.. particularly if you are not well practiced at shooting with both eyes open

Second weapon.
p226. very accurate, the double action allows for the weapon to be carried charged, but safely I carry mine in a drop leg.. with the pistol slung at just above mid thigh.

Third weapon.
Rubber Knife... for "wet work" if the rules allow.

I have seen the shotgun put to very effective use.. but you have to be skilled to use it due to the length of the weapon

Folding stocks.. many like them.. but they are overrated. to effectivly deploy a weapon with a folding stock you have to unfold it.. and they are usually not as robust as a fixed stock, and shorter. Anyone who shoots their weapon with the stock folded... 1. has probably never fired a real weapon. 2. does not know how to shoot, or the principles involved.
Folding stocks are for making the weapon compact when not deployed, not for for "shortening it for CQB"

Really, if you know how to shoot and move well you don't really need anything other than a pistol for CQB.
CQB is likely the most accessable of the airsoft activities. 1 pistol, 2 mags and practice .. and you can be just as effective as anyone on the field.
Upgrades don't mean squat in close quarters.. skill at arms is the factor.