I haven't played any PC games as much as I played Diablo and Diablo II, but I'd say the Baldur's Gate games were pretty good. LOTRO is a very, very good MMO.

Just bought Crusader Kings II. Think it was here or somewhere else that led me to take the plunge, but it was at a mark down price, so too good (and cheap) to pass on.

A lot of it depends on what genre you're looking at.  Management/Construction games like Dwarf Fortress, Minecraft, and the aforementioned X series all have massive amounts of replay value -- if you have the time and desire to play them at all.  But even Minecraft isn't for everyone, and many folk simply don't find it entertaining, while even X3: Albion Prelude and especially Dwarf Fortress are mindbogglingly difficult games to actually play and especially enjoy.

My brother could (and has) played Just Cause II for something like four hundred hours; if you're into Grand Theft Auto-style games but want something a little more explody, it's very hard to beat.  Other folk... well, I don't understand Halo or Counterstrike experts, but they certainly exist.  That said, some general ones :

Deus Ex.  If you haven't played this game and can do FPS at all, you owe it to yourself to run through at least Liberty Island.  It's not the end-all be-all game that its more diehard fans propose, but it's still damned good.  Likewise, Vampire: The Masquerade: Bloodlines until at least the end of the Hotel.
Minecraft, especially with The Aether mod, and Terraria.
The Elder Scrolls, particularly Skyrim and Morrowind.  They're fundamentally flawed games at a couple mechanical levels, but the sheer amount of content and variety of background is amazing.  You will needs mods.  You will want the mods.  There are folk that have added entire new provinces worth of stuff through mods.  So, both in terms of time spent playing through the game at once, and in total replays, they're probably going to beat anything that isn't an MMO.
Fallout : New Vegas and Fallout II.
Any one game from the STALKER series.
Portal (not Portal II).  Not the gimicky voice acting and songs, although those help, but because it's the single distillation of level design that really needs to be there.
Homeworld : Cataclysm and Freelancer (or Freespace II mods).  The peak of their respective, and largely defunct, genres.
Are all games that I could pick up and play, even if I'd beaten them the month before.

Okami, Legend of Zelda : A Link To The Past, Heavy Gear II, Front Mission III, Bujingai, System Shock II, and a Harvest Moon game probably go into a separate category; they're enjoyable enough to play more than once, but you do need time between playthroughs to keep them from becoming trite.

Portal 2 has the level editor, ergo potentially more replay value than Portal 1.

Quote from: APN;568242Just bought Crusader Kings II. Think it was here or somewhere else that led me to take the plunge, but it was at a mark down price, so too good (and cheap) to pass on.

If you like it, you'll definitely want to buy the Sword of Islam DLC, which is the meatiest by far of all the add-ons. It let's you play Muslim rulers and nobility whose mechanics differ from Christians in several respects. Also enlarges the map towards the southwest to incluse some sub-Saharan Muslim lands.

I seem to be spending quite a bit of time playing Civilization V:Gods and Kings. No two maps are the same, ever. So the game always plays out differently, and there are soooo many Civs to play. Just wish there was an accessible SDK to add in even more cool stuff. The Civ 4 SDK was awesome sauce for adding in bonus content!