Counterstrike, Diablo II
Most of the games on this list are open-ended world builders of one sort or another. Counter-Strike (CS) is a notable exception. The original game was a Half-Life mod that became hugely popular in the years between HL and HL2. In 2004, Valve built a revamped version of the game, appropriately titled Counter-Strike: Source. Both the original game and the CS:Source remake remain playable through Steam. A sequel to CS:Source, entitled Counter-Strike: Global Offensive, was released this past summer.
Counter-Strike's game modes, weapon damage, and a number of other factors depend on which version of the game you're playing, but certain conventions are common to all three titles. Death is permanent in the majority of maps -- once you die in a round, you stay dead, rather than respawning. That tactic is the core of the CS experience -- weapons are purchased from round to round based on your previous performance and there are no game-changing weapons you can unlock via kill streaks or other events.
What sets Counter-Strike apart from a number of other popular franchises, like Call of Duty, is the launch of a new game doesn't cannibalize the previous titles. Thanks to Steamgraph.net, we can show you objective data to prove it. Steamgraph's data only extends backwards through May 5, 2011, but that's long enough to establish short-range trends.
CS:S's daily rate dips slightly on the day CS:Go is released, but recovers thereafter. The original Counter-Strike doesn't budge at all. This makes sense, if you're committed to playing a ten year-old game already, you probably don't care much about a sequel launch.
CS:Go's daily rate settles just above the 20,000 mark. The net number of people playing a Counter-Strike title has risen slightly since the new game's release, from ~101,000 people pre-launch to 122,000 today.
Now let's compare that to the popular Call of Duty franchise.
This graph neatly summarizes why Counter-Strike is on our list of most addictive titles, while the hugely successful Call of Duty isn't. Each CoD title rapidly cannibalizes the game that came before. When Modern Warfare 3 debuted, the total number of CoD players hit 128,794. By the time Black Ops II hit the streets, this had dwindled to ~53,000. Black Ops 2's impact sent total players soaring back to 105K, but dwindled quickly thereafter. Right now, the total number of people playing one of these four titles is just over 70,000.
Diablo II doesn't hold my personal record for most hours played, but it's the only game that sucked me in so deep, I became addicted to improving it. I've written about that effort before, and I won't rehash the entire history here, but the game struck an amazingly good balance between addictive playstyle, an explorable world, innovative quest structures, and interesting gameplay.
It built perfectly on what had come before in Diablo but expanded the canvas from a single world in the same town with varying dungeons to an entire world of brilliant colors, evocative music, and differing architectural styles.
Who could forget this musical gem?
Diablo II introduced Blizzard-hosted servers meant to reduce player cheating, after numerous flaws in Diablo had effectively destroyed the game's multiplayer. The Open vs. Closed Realm system succeeded at this, at least for the years following the game's release. Blizz also introduced a new permanent death option via Hardcore mode, and designed many character abilities to function best in parties.