The best quite pistol out there is probably the tm MK23, get ready to spend a ton if you want one though. EBB's are also really quiet but they kind of suck.
Another option is a non blowback pistol (like the MK23), they are somewhat quieter than their blowback counterparts. A socom gear gemtech oasis/kjw mkI would be a decent option.
Edit: What they said ^