From a commercial standpoint it's what everyone has said so far..
Counter Strike and Dota and quite easily so.
I started with Beta 4 on CS in 1999 before I fell off some time after beta 5.2.
Also played Dota All-stars for WC3 for many years, but being a big Starcraft nerd I even played Aeon of Strife which was the original dota :P Even though not at all as fleshed out as DOTA became.
It was hella fun, I prefered the Quake CTF my self, but the hours we played these two mods on LANs is pretty staggering.
Counter Strike and Dota and quite easily so.
I started with Beta 4 on CS in 1999 before I fell off some time after beta 5.2.
Also played Dota All-stars for WC3 for many years, but being a big Starcraft nerd I even played Aeon of Strife which was the original dota :P Even though not at all as fleshed out as DOTA became.
Quake Team Fortress. It was pretty much the first big multiplayer mod AFAIK.
It was hella fun, I prefered the Quake CTF my self, but the hours we played these two mods on LANs is pretty staggering.