Left 4 Dead 1. 9/10 Most amazing game ever, and you can still find me online weekly. Played it from the moment it was released, 3000 hours total and counting. Drawback is online you will only find players that know whats what, no noobs, so very inaccessible to anyone new to the online modes. If you dont own it, dont buy it, you will just get kicked in mulitplayer.
Left 4 Dead 2. 11/10 Everything that made the first one good, then add some more. Plenty of noobs to make miserable in versusmode. Steam says I have 1200 hours so far. I play a round every day.
Evochron Mercenary. 8/10, be whatever the hell you want to be, pirate, slaver, trader, warlord, merc, just easier controls than X series of games, also more limited when it comes to building your empire.
X3 Terran Conflict & X3 Reunion. 7/10. TC has an overhauled interface that was supposed to make it easier to new people. That failed. Hard. IF you get past the hard as hell controls you are left with your very own playground where you can do... well anything you fancy really. Points off for the most user unfriendly UI right after Eve Online, the graphics that are starting to look dated(sprites for explosions instead of particles for example) and the lack of online play.
Silent Hunter 4. 8/0. Sneaking up on some kraut or Nip warships, smashing them with the iron fury of US manufactured torpedoes after carefully calculating a firing solution, doing as much damage as you can then avoiding the destroyers hunting you with a crashdive and some hard manouvering. Sweaty palms guaranteed. If you are a casual you can leave all the work to the computer but that is no fun.
Which brings us to Arma 2. Solid 8/10, well usually. If you don't get the whiny 14 year olds that crash all the jets and leave you without airsupport, or the 2 year olds that whine about how the controls are stiffer than COD, and then I'm leaving out the idiots that think it's a great idea to stand still in urban area's in a tank while ****'s flying. You'll usually only find these on Domi(Domination gamemode for public play). There's boats, Chinooks, attack helicopters, tanks, Ac-130's, artillery, Scuds, laserdesignators for pinpoint bombing runs, etc. Did I mention you can play it for free? To be honest, animations and movement require work, vehicle physics need updating, and other stuff, but the soon to be released Arma 3 has all of that and subs, underwater combat and demolitions, better graphics and some other goodies. Serverwise it's not all that great, you need hellapowerful hardware to run any decent amount of players(no proper multicore support on the dedicated server). Play Arma 2 free if you are curious but do not buy it even if you like it. Buy Arma 3 instead.
I also saw some people being pleased with Halflife 2, well they released Black Mesa mod, HL1 with HL2 visuals. And it is free too. All you need is a Steam game, but the Steam SDK 2007 will suffice. So just make a Steam account, go to tools, select SDK 2007, install it for free, fetch Black Mesa here, and enjoy HL1 in an all new jacket, no paying, completely legit. The catch? It was supposed to be finished february 2012 if I remember correctly, so it is safe to say they are a bit behind schedule. But they released what they had so far, only last few levels are missing.
Payday: The Heist. 8/10. Great fun online. I still play 2-3 heists a week. Robbing banks, street shootouts, stealing gold from an armoured van, a big diamond heist. It's fun. Some players are complete ******* and screw you over bad at times, which can be immensely frustrating, but is not a fault with the game per se. The bad: I find the animations lacking and needing a little more polish, it can be too hard at times, and they borrowed from COD a bit(too mcuh?) and it shows(special cops like dozers and shields), but nothing that would make me ever stop playing this little gem.
These are the games I play at least once a month. I do play others like Syndicate, Portal, Stalker, Metro 2033, but once you played them it's pretty much done. I tend to not play offline games a lot. Get's repetitive fast, and the AI can be learned and accounted for. Human opponents are unpredictable and can surprise you, much more fun.