Definitely Deus Ex or actually, System Shock 2 or err, Deus Ex. Maybe System Shock 2. Aaarrrrgh....can't.... decide....

Deus Shock 2 < without a doubt.

Might and Magic 6 and 7

My first RPGs, the games that sucked me into the fantasy-RPG genre, the games that made me into a fantasy-RPG fanatic :)
Finished them like 1000 times already, and I'm pretty sure I will finish them at least 1000 times more again.

Fallout Tactics and Arcanum

Neverwinter Nights. Sure, there are games with better stories, mechanics, etc, but I haven't found any games with as much to offer overall. Neverwinter Nights 2 tried, but I don't think it ever reached the same heights (though it would still be on my short list of favorite games).

For the last year or so - Faster Than Light. I hardly play anything else, recently. If classic - Jagged Alliance 2, followed by Master Of Orion (1 or 2).

Deus Ex. I have uninstalled it occasionally, but it always ends up on my computer again. I first played the demo/preview version of this game back when it was released. My dad worked at a computer store and got it for free somehow. I wasn't very old and didn't understand half of it, but I loved it. I played at least a dozen times. It wasn't until years later that I got the full game. It was simply incredible. I've played the full game at least four times, and while that may not sound like much, for me, there are some games that are so incredible they just can't be played too often. This is one of them. But every time I do play it, it is no less incredible.

I think, though, I may have discovered three new games that will be one I come back to time and time again: the Thief series. I bought them a year or two ago and thought they were awesome. I've already played the first one twice, and I have a feeling I'll play it many more times in the coming years.

Gothic 1+2
and Fallout 1+2.

I don't know how many times I played through these.
Best RPGs ever made.
Inquisitor may get into that list though, it was really good.

Thief. Because perfection! Because this is also the game that made me appreciate monster infighting - which is why I usually play Thief like an FPP real time strategy game. Because the sound design, to this day, is second to none. Because of the uniquely creepy atmosphere and amazingly compelling setting. Some people don't like the undead missions. These people are wrong. Oh, how I wish gog would add the vanilla version of the original Thief (not to mention the outstanding German and French versions)...

They are sold together here but they are distinct games to play. Meaning you choose to play either Zeus or Poseidon and you dont play Zeus under the Poseidon rule set, etc.. So I let myself off on a technicality!

Pharaoh was soemthing of a disappointment for me. Perhaps because I played Zeus firts long before I discover Pharaoh, but it was one of those games I didnt enjoy going backwards on. Caesar is probably the only "historic" setting I enjoy more than Greece and I did love that game, but the interface and tools are sooo much better in Zeus.

Disciples 2 definitely. If I owned fallout 1&2 on gog as well, then that would be my answer instead. As soon as Bladerunner comes to gog, then that will become my answer :)

Could you please develop a bit ? What do you mean, here, with monster infighting and FPP real time strategy ? And what do you mean with vanilla version instead of gog ?

I ask this as a Thief owner who hasn't installed it yet.

My favorite for the games i own is easily The Witcher Enhanced Edition..love this game

Dark Project instead of Gold or without NewDark?

For me that will be Zeus. For games not on GOG, that will be KOTOR1 and 2 (specially 2 after TSCLRM).

Some of the different types of enemies you'll encounter in Thief are hostile towards each other. You can use this to your advantage and lure attackers into enemy territory. Of course, calling this type of gameplay "real time strategy" is a bit of a stretch, but it's fun nonetheless. Thief is a game that really encourages experimenting with the environment (and its inhabitants).

As for the vanilla version: gog sells the "Gold"-version, which adds three levels to the game which weren't included in the original release. While I personally like these three new levels, they to tend to break the pace. One of the added levels, the Thieve's guild, has become infamous for its convoluted level layout and is widely considered one of the worst levels of the original Thief games. Playing the original version is a different experience from playing the Gold version and it would be cool if you could choose which one you'd like to play.

Nice catch, I completely forgot about NewDark, probably because I still have the non-NewDark gog-installer. But, yeah, I was referring to Dark Project.