Any of them, you just need to upgrade our gun possible (new inner barrel, spring, etc.)
A good pick for your first gun should be your favourite gun, or in this case your favourite CQB gun.
I use a P90 for outdoor play.
Also get age verified if your over 18(more on that in the FAQs), if not wait till your 18. This is a standard response beginners get all the time so please don't take offense to it.