I'm doing a sneak playthrough and I'm looking for a good gun, preferably a pistol or anything silenced. Does anyone know of a good gun or guns that can be modded with a supressor?

You don't really need a supressor. It's all about your sneaking skill. You can even use a rocket louncher. NPC doesn't react to the sound of weapon, they only do react on sight or dead bodies. Well... dead bodies attract them in the wrong way:

two soldiers guarding the gates. You kill one of them silently. Another guard doesn't see you, but he can see the body. Guess what he's thinking about it? 'Oh jeez, a dead body! I better loot it before anyone do it before me'. So he goes and loots a body. And then keeps on guarding the gate, waiting for you to kill him.
They are that stupid, yes. No wonder NCR got themselves irradietad at the Searchlight.

Actually that is wrong npc ARE attracted to sounds. Shoot at deathclaws with the .50 cal. Even if you blow off the head of one of them with one shot if your sneak isn't high enough the other ones will run over and rip your head clean off.

I use Gobi Sniper for .308 rounds - powerful, good resilience, but not silent. I use Ratslayer with AP 5.56 for sneaky sniping.

Best weapon would be the mr. sandman perk and lot of disguise clothes

Highest damage silenced ranged weapon is a silenced Sniper Rifle.

Other weapons that can be silenced is the 10mm gun.

Melee weapons are also silent.

They are not an NPC. They are monsters. I described NPC AI.

Apart from the screams of the poor [censored] as you sneakly slice and dice him with said silent weapon :rofl:

Not sure how well it would work on higher difficulties but if i had to choose three good silent weapons i'd say a modded varmint rifle (or the ratslayer which comes with all three mods) for weak enemies at long range, a modded sniper rifle for tougher enemies at range, and either a modded 10mm pistol or modded 12.7mm pistol (since for whatever reason the 9mm pistol does'nt get a silcener).

I used Anti-Material Rifle for long distance, Ranger Sequoia for Medium, and Riot Shotgun for short. If you want to stay hidden then you can have a Fully Upgraded Sniper Rifle, Silenced .22 SMG, and a Fully Upgraded 10mm Pistol (Or silenced .22 Pistol)l. Melee weapons are another choice as well since they are silent.

Apparently the 12.7mm SMG has a silencer mod available for it, although personally I was never able to find it.

Has anyone else had any luck finding this gun mod?

http://fallout.wikia.com/wiki/12.7mm_SMG

12.7 silenced pistol or silenced 12.7 SMG, if you want something small and for it to count towards The Professional Perk.