Since their is a "Best game you ever played" thread why not best mods you ever played.
Mods give us new weapons, levels and alot more.
The Specialists 3.0 (Half Life One) TS 3.0 Final Montage
First Person or Third Person Views
Many Weapons including SMGs, Rifles, Handguns, Shotguns, Swords and Knives plus Attachments such as Scopes, Silencers and Lasers.
Many levels inducing one from the Martix titled Lobby.
Realistic Recoil on Guns
RP and Deathmatch both fairly well populated. Even after 6 years or so.
Playing for 5 years and it is still awesome.
Lance 2 (Gears of War) Lance 2 Gameplay
Added Bots to Multilayer.
5v5 or 6v6 From the standard 4v4 online matches.
Many amazing custom maps that some are better then the ones that came with the game.
Bots can be hard and moderate.
Lots of fun when your internet is not all that great for versing actual players .