My personal favorite is the normal Sandbox mode, as its raw simplicity entices creativity and (in some ways) ingenuity, allowing the player to pretty much do as much as their imagination will take them. Sessions can go anywhere from building, to roleplay, to deathmatch, to even such simple things as goofing off with friends with integrated Peer to Peer servers. So much can be accomplished within this gamemode, with many addons stretching the limits of the Source engine itself! It really shows how dedicated the fanbase of this game can be, and it's all thanks to this one gamemode.
