Gun choice depends on map. For close-range like CS_Office, I would go for Deagle/USP for pistol, P90/Mp5 for SMG, Galil/M4 for Rifles. For more open space, same for pistol, Mp5 for SMG, AK/M4 for Rifle.
Galil is under-valued rifle in close encounter maps like CS_office. It spray area is something between a M4 and AK, ROF is faster than AK & M4(not confirmed). It's great weapon to spray down the hall or quick turn on a corner. Even in more open maps, Galil is good for those who likes to rush in.
P90 is also great in CS_office because a short burst of P90 is as accurate as Mp5 and 50-round clip is excellent for suppressing an area or mad surprise rush.
Shotgun is fun, if you like the challenge. Auto-ShotGun is great way to go against 1 or 2 opponents.