Jagerwerks does great slide work as does Loki Tactical. Prefab though I'd say Zev, Southwest, L2D.
Barrel/Comp: Primary Machine stealth comp, Agency barrel/comp, Texas Black Rifle Co, Mayhem Syndicate, Black List, Faxon... too many to name really. Barrel is a barrel in my opinion, some just have a bigger bling factor than others.
Triggers: Agency, Apex, Overwatch, Tactical Trigger/Kineti-Tech which is what I am currently using.