A variety of things will do it for me. I have a bunch of old games that I still play regularly. A few of those are:
Silent Hunter III
I'm a bit of a sim-nerd, and I'm also a bit of a military history geek as well. Very few games have ever immersed the player in the action the way that SHIII did, especially with the Grey Wolves 3.0 mod. It's so good, that I'll still take it over either of the sequels any day of the week.
IL2:1946
As above, but in addition there is still a fair sized online community for the game. It's also one of the few online communities where "n00bs" aren't considered the scum of the earth and are actively encouraged to learn and improve their skills. It's a good mentality for a community to have I think. And besides, there's far more satisfaction from scoring a victory against a competent pilot who makes you work for it instead of against a novice who's flying in a straight line all dumb and happy. It's not uncommon to see more experienced players taking newcomers under their wing (see what I did there?!) and teaching them the finer points of aerial combat.
It's also pretty much the last "serious" WWII era flight sim made, as the genre seemed to slip quietly into obscurity around this time. There have been a few half baked efforts since then, but nothing that came even remotely close to the scale, accuracy and sheer playability of this. Combined with a healthy modding community and literally thousands of fan made missions, campaigns, aircraft skins and whatnot it's literally endless.
Rome: Total War
Apart from being a military history geek, I also get a bit of a chubby for classical history. I also enjoy strategy games, but because of the classical setting, Rome is still my favourite game in the Total War series. I also have this one modded, with the Roma Surrectum II mod. It's a complete rewrite of the game from the ground up and I feel confident in saying that no matter how good it turns out to be, Rome 2 will still not be able to touch RSII in any area other than graphics. A bold claim, true, but anyone who has played RSII probably knows why I've said that.
Grand Theft Auto: San Andreas
Because it's still the best GTA by a country mile. My PC copy has gone missing somewhere, so I was delighted to spot it on the PSN store for a very reasonable sum recently. Grove Street for life, Yo!