I don't like top game lists because you take nothing away from them. So I'm doing something a bit different in listing a random amount of games I keep on returning to years later. There is no real requirements except for the game being replayable. Lets start:
Unreal Tournament 2004: This game is the last truly great arena shooter. The market is flooded with Call of Duty clones which I have absolutely no interest in and only turns me off of the FPS genre. Whenever I get a strong urge to play a shooter this is my first choice.
The online community isn't the strongest but the bots more then make up for it. If there is one thing I like about this game it is the bots because they actually rival and sometimes pass humans in skill. The game even has a feature to let the bots adjust in difficulty so it is never too easy or too hard.
I love the variety this game offers. It shipped with something like 90 maps that can be played to death. How many did Call of Duty 4,5 or 6 launch with? I bet its less then 90 maps put together including the ones you have to pay for. You don't pay anything for user maps here.
The weapons are some of the most unique in an online shooter. There is a gun that shoots slime, one that discharges an electric pulse to do massive damage, a machine gun that shoots grenades, a flak cannon that blows anything up in close range, and a minigun that can really hurt people or vehicles.
Modes range from deathmatch, team deathmatch, capture the flag, take objectives and king of the hill.
This game beats Quake 3 in sheer replayability. Yes I went there.
Guild Wars: Being in the RPG (not to be confused with the MMO but could be could be classified as an MMO-like) genre you would think these games weren't replayable in the least. Well that couldn't be further from the truth. Everything in this game is made to keep you playing without grinding. I hate grind, like its the reason I can't stand an entire genre of games and most likely won't until it shifts towards what Guild Wars stands for. There are 2 ways to play the game depending on preference, PvE or PvP.
In the PvE you go through a mostly linear story to reach a conclusion. While the story is linear there are loads of sidequests (that matter to the lore) that will have you exploring the world. Exploring is so great because the developers hid so much stuff off the beaten path. 1400 hours into the game and I still find new stuff. Best of all this isn't linear in the least so it will keep you playing. After finishing the story you can go through the missions again in a harder mode or play a different story. There are 3 stand alone stories and one expansion which offer 150-200 hours of gameplay for the first go.
PvP is a whole different game. It focuses on using tactics and good team composition to beat the enemy. To continue with the no grind formula you can make a max level character with the same gear everyone else uses. The only things that need to be unlocked are skills which can actually be bought from the in game store to save some time for like $40. If you wanted to skip out on the PvE stuff just get the skill unlocks and jump right in, I believe you don't have to buy the stand-alone games this way (correct me if I'm wrong).
Leveling and gear are what make the game different. The max level is 20 and is reached very quickly in all but 1 campaign with the max gear obtained shortly thereafter. The idea is you spend all the time playing the game for fun and not to obtain items or that next level. Think of it the antithesis for MMOs.
Another unique feature is the profession system. When making a character you choose a profession like any other game. Later on down the road you take a secondary profession whose skills you can add to your existing one.
What really set the game apart is the skill system. Ever played Magic the Gathering? Well this game has skills that act like the decks from that game. They can be changed out at any time in an outpost from a large variety of skills and make every build feel unique. There are the cookie cutter builds if that is your thing but I like off the wall builds.
An example I'm messing with right now is a Ranger. Because of the profession system and the wide variety of skills my ranger is able to do the normal get a bow and kill things but she also has a warrior mix. The warrior is a melee specialist but has a thing called shouts. As you hit with attacks adrenaline builds up and lets you use these shouts. The idea is to use a skill called Barrage to hit a lot of enemies, use the adrenaline gained to use shouts that buffs my party. A build I did in the past was instead of the shouts I used necromancer debuffs that deal damage to itself and all enemies near it when hit for massive damage with Barrage.
The final thing of note is this is a party based game. Past the newb island everything is done in a group no matter the game mode. It is a bit hard to find players but heroes introduced int he Nightfall campaign make this easy. Think of heroes like computer controlled party members (also available as henchmen) that you can customize skills (more builds!) and weapon. Henchmen don't have any of this and tend to suffer from builds that don't have good synergy.
This is an online RPG that should really have set the precedent for others and is the most engrossing in its genre.