Such a nostalgic question. These are not arranged in any particular order. I'll list ten, but there are plenty of other favorites.
1.) The Marathon Trilogy
These three games will never get old for me, and their story rivals some of the ones presented in todays games. Basically, it involves a Human 'generation ship' being attacked by alien slavers and the onboard AI's go rampant while you play as a supersoldier. Created by Bungie, it's a spiritual ancestor to the Halo franchise.
FYI - Marathon: Durandal, the second game, is available on the XBox Live Arcade with a free trial to those wanting to check it out. I'll warn you now, though. There is no jump button. Yes, it's that old.
2.) Star Wars: X-Wing Alliance
Perhaps one of the best combat flight simulators out there, X-Wing Alliance was a blast. Featuring a lengthy campaign and the ability to create custom battles, there was plenty to do. But the best part was the multiplayer community. Engaging in clan wars, dueling or just trading fire with random players was unlimited fun.
I'd kill to see a new game on par with this, but I fear that in todays society, nobody could be troubled to pick up a $20 joystick. Nor does anyone want to deal with the complexity of such a game where you had to adjust ship power and shield levels.
3.) MechWarrior 4: Mercenaries
This one's another simulator, but with mechs. The single player campaign was pretty cool, but again, multiplayer was what this game was all about. With dozens of individual mechs, you could customize them on a detailed level. From paint schemes, to weapon loadouts and armor distribution. It was epic.
Hopefully MechWarrior 5, which is in developement, proves itself worthy of the franchise. I'd be extremely disappointed if it turned out to be like MechAssault, which sucked.
4.) The Halo Series
The days of Halo: CE LAN parties are a fond memory to me. Probably one of the funnest co-op games I've ever played, multiplayer was great. But it didn't compare to the action brought on by Halo 2. Halo 2 had a terrible single player campaign, and it had too many exploits in multiplayer, but it was fun. Having a remake of the famed Blood Gulch to fight in online was really epic, too.
Halo 3 was a surprise in that it didn't suck as much as Halo 2. The campaign was pretty cool, and 4-player co-op was pretty awesome. However it easily has the best multiplayer out of the entire trilogy. My only qualms with this game is that they could have done away with the annoying Cortana/Gravemind interrupts during single player and that they didn't put a Blood Gulch remake in to multiplayer.
5.) The Mass Effect Series
In my opinion, the best sci-fi RPG I've ever played. This dialogue driven game is mixed with plenty of action. Each character has unique and interesting personalities, the environments were awesome (aside from the cookie cutter underground areas of the game) and the combat was excellent.
I could rant on about this game further, but I don't want to write a book about it. For those interested in the series, you can get away with buying Mass Effect 2 without playing the original, however you're missing out. Aside the fact that you can import your Mass Effect 1 character to the sequel, the first game is nearly as good. The combat isn't as fine tuned, but you have my recommendation.
And I'm probably the only person who didn't mind the Mako from Mass Effect 1. Driving around vast stretches of an alien world was just awesome.
6.) Dead Space
The most underrated game I've played. Few developers have succeeded in creating a survival-horror game that works. Resident Evil was good, but that particular franchise has gone downhill. Even though 90% of the game takes place within the bowels of a massive mining ship, the developers offered the player plenty of unique environments.
You play as a simple engineer who must fight his way through hordes of monsters, known as necromorphs, and unlock the mysteries of an alien artifact that the miners had discovered which caused the crew to...change. Using little more than mere engineering equipment.
A sequel is on the way which I expect to uphold the same quality of this amazing title. But this game is worth a buy to any sci-fi fan. Seriously.
7.) Diablo II
While the first game was quite enjoyable, this sequel dominates. I can only imagine how many hours of this game I played during Middle/High School and how many Hell runs I've managed with my friends. It's simply a timeless classic, and I still give it a go every now and then. The expansion was pretty epic, too, and is definitely worth a buy.
I loved being the Necromancer, and I'm disappointed not to see a proper analogue in Diablo III. (A.K.A., 'WoWablo III'). I seriously thought the Witch Doctor was a joke at first.
This is the definition of a timeless game. Although archaic by todays FPS standards, this classic is still played to death by a legion of dedicated players. I may get frustrated at how that guy snapped off a headshot with an AK-47 from across the map or how that grenade just dropped out of nowhere and blew my ass away, it's a good game for 'shooting the shit' fun. Casual and pro gamers alike can enjoy this game.
Steam recently revamped the game, adding achievements and stats. So this games pulse is still quite healthy and strong. Plus, people actually run servers that aren't Dust or Office for a change.
FYI - If anyone plays this (I did make a thread about this game, so disregard if you have already said so there), let me know. My playstyle is very lax, and I don't take the game too seriously, but I'd be glad to gun with a few folks from Heresy. My Steam ID is Mjolnir73
, profile name [SDETH] Drunken Swede
9.) EVE Online
My one and only favorite MMO. Though I've played World of Warcraft, D&D Online and Star Wars Galaxies (before Sony fucked it up), this is the only one I've stuck with. EVE Online is a stupidly complex space flight simulator. The learning curve is steep (See the photo after this for an idea), but after a week or two you will 'snap' in to it and be good to go. Many are discouraged by the difficulty at first, but really after you begin to grasp the game mechanics it's a piece of cake.
Even people who don't play this game find it interesting in the fact that it is all hosted on one server, with around 20,000-50,000 players on at any given time. On top of that, there are over 7,000 individual star systems and 66,000 planetary bodies. Yes, the game is that big. Not to mention the wormholes, asteroid fields, deadspace facilities and space stations.
The ships available to you cater every aspect of the game. From combat to spying to industrial purposes. Players can fly anything, given they have the training, from shuttles to mining barges or battleships to 10+ kilometer long Titans. There are dozens of ship classes, all suiting different needs and each can be fitted with a dizzying array of equipment.
The story is thin, as the game is more based on PvP/player empires which fight for control the 'null-security' space, but cool. Basically Humanity discovered a wormhole in our galaxy that led to another. They began colonizing this galaxy and the wormhole collapsed, trapping these people in an alien corner of space. After a technological dark age, things returned more or less to 'normal', with several distinct factions controlling different parts of space. Of which 4 are available to players, and a 5th for game masters/developers. There are also various NPC factions, mostly pirates, who infest the galaxy at large.
Let me know if you play or are interested in playing. I might be able to hook new players up with a 21 day trial and some kredits.
10.) The Half-Life Series
I enjoyed the original game more so than the sequels, but that's not to say the Half-Life 2 games are bad at all. The first was just a great classic that I've played more times than I could probably count. They need
to put that on the Xbox Live Arcade, now that I think of it.
Regardless, in the original you play as a scientist who unknowingly starts an inter-dimensional disaster. Aliens pour in to the secret research facility and you basically fight your way around trying to stop the invaders from taking Earth. Halfway through, a US Marines spec-ops force attacks the base in a cover up operation. It's pretty cool.
The sequels are fun too, and you really don't need to have played the original to enjoy Half-Life 2. The characters do a good job of telling the player what you need to know about it. So don't worry about picking them up.
FYI - An independent group of developers are remaking the original Half-Life using the Half-Life 2 Source engine. They are nearing completion, and guess what? It'll be completely free via Steam. You need only to have Half-Life 2 installed. Here's a trailer for it. Those familiar with the first game should immediately recognize a few of those locations and will perhaps understand why there is a cockroach at the end.