Garry's mod. 
Vanilla G-mod isn't bad really.
Sadly..no server really is Vanilla flavor...they more often consist of a cacaphony of poorly made, inefficient, download heavy, texture-bug-ridden, mods and gamemodes almost certainly designed and implemented in an attempt to alienate all but the most diehard players.
The source engine was built for typically small levels with minimal details and a limited number of objects...so what do players do? they stuff in as many models, textures, scripts as they possibly can into the engine, and even then they aren't done...nooooo. After that they go about creating massive, largely empty, levels easily twice as large as the program was designed to take and fill them with poorly made custom textures, secret rooms full of buttons and a couple of poorly hidden easter eggs.
I can't blame Valve for the ball of stuff that is now Gmod because the Source engine isn't that bad when it is used for what it was designed to do, nor can I blame Garry because he has at least tried to fix the issues, but it really has degraded into a horrible hodge-podge collection of minigames and pipe-dreams.