Depends on exactly what is going to be your typical CQB scenario to some extent, and not everything which follows with real firearms will be relevant to airsoft equivalents. As I'm sure you are aware, the MP5 (usually the very short MP5K with a fore grip) used to be the de facto standard for CQB weapons, which is why it was and - in many cases - is still used by so many elite forces, hostage rescue teams etc.
However, in its real form, the MP5's 9mm round does lack penetration against body armour, which has led some forces to switch to using - or at least supplementing team load outs - with weapons which fire ammunition more able to defeat kevlar etc. But of course with airsoft, the weapon type is largely only cosmetically different since they all fire the same 6mm BB at about 350fps, thus there is no real reason to forgo the compactness of the MP5 over something which in the realm of real firearms, would warrant consideration. That is unless you want to take this thinking further...
With CQB, the range is by definition of course very short, short enough to warrant a pistol, and in fact some airsoft CQB sites specify only being allowed to use pistols in confined spaces such as stairwells anyway. There are other advantages conferred by this of course, notably the fact that you can operate a pistol with one hand (either in fact, which is good for coming around corners both left handed and right handed), leaving the other free for using flashbangs, smoke grenades or indeed a torch which can more easily be manipulated and probably switched on and off quicker, which is good for light discipline too.
Of course you could go for both a pistol and a submachine gun, and if you did embrace the MP5K's compactness, then a believable pistol combination to use with that would be the FN Five Seven, since the Five Seven's 5.7mm round complements the 9mm ammo of the MP5 by virtue of the Five Seven's infamous ability to punch through kevlar armour. Again, this calibre is of course irrelevant for airsoft ammo, but it is a believable and thoughtful combination which would reflect the choice of many in the real world who might have to engage in CQB, not least because it has a 20 round mag as standard and a 30 round extended mag available too. The real thing has all the usual CQB features one might expect, concealed hammer, underslung rail, threaded for a suppressor, can be switched to left handed use for the mag release, etc.
Beyond these aspects, for a 'serious' airsofter, the fact that the Five Seven and the MP5 are both available in robust RIF form, means you can get some decent kit with that combination.
