You haven't installed it yet!?
Granted I only know you from this silly forum but from that I would wager you'd love the Thief series. It's got a little bit of everything and more than that offers you the opportunity to be creative with multiple ways to approach any level or challenge. Call me nuts but I get the vibe that you'd thrive on that.

I have, between my post and yours.

Now, to derail it further, is there a partch I should install to update it graphically without replacing the original designs ?

I honestly don't remember if I did or not. TBH I bought it here because the cd copy I got a million years ago was getting to be too big of a pain to run; I was just so grateful I didn't have to go in and start editing code.

On every computer I own I have installed Hitman 2 & 3 (available here) as well as Hitman Blood Money since they came out.
I also have Dungeon Siege 1 + both addons and the Tomb Raider series too. I might not play them every year, but it is always nice to know they are there.

Toss up between The Witcher, Fallout 2, and Divine Divinity. Admittedly, since getting a new computer, I've only beaten Witcher, but all three really did bring back to video games after a period of disinterest. Not total disinterest, but I could never justify foregoing studies or stuff like that until I got Fallout 2, and then again with Divine Divinity. The Witcher really is amazing as well, one of the few modern WRPGs that I thought was really, really good. The Witcher 2 does not count because I bought it on DVD.
Not too "bar none," but what the heck.

MOM 2 or SMAC

Vampire the Masquerade Redemption

It would be American McGee's Alice if it ever got released

It's extremely rare that I replay games and the only 3 I can currently think of are not on GOG.

1. The Secret of Monkey Island (only adventure game I've played more than 3 times and never get tired of)
2. Transport Tycoon (most criminally addictive managerial game ever, only Sim City 2000 can compare)
3. Diablo 2 (of all the low down primitive games this one is probably the most gratuitious and the most addictive)

Out of those 3, I only truly love Monkey Island, the other 2 are more like drugs rather than games.
The original X-COM comes in 4th in terms of how often I get back to it despite the fact that I like it more and think it's a better game than the ones I've listed above.

As far as the GOG catalog is concerned, my top favorites are Incubation, Little Big Adventure and Worms 2. But I haven't returned to any of those games since I originally played them.

I'm guessing you never played Act III as a barbarian.

Hmm. I can't really answer this, as I haven't played all the games on GOG :P

Out of those I've played, though*, it's a toss up between Eador: Genesis, and the Baldur's Gate games. (I'm treating BG1 and 2 as one game by technicality, since I played them through BGT :P) While I haven't "come back" to either of them, in the case of Eador: Genesis I din't "come away" from it for many months. It hasn't been that long since I last (and first) played BG to completion, so I don't want to start another character, but of the games on GOG I've played, it's undoubtedly the one which will continue its impact on me the longest. And not just because it gave me the idea for the dnd character in my current campaign.

I haven't really "returned to" any games yet, because GOG has so many excellent games and replaying a game is never as interesting as playing a new one of the same quality, at least for RPGs.

*Which doesn't yet include PS:T, which may well end up surpassing them

I've even played it as a pure melee amazon (spearazon) and prior to having a valk at lvl 30 on nightmare those little jungle stinkers are even more annoying than as a barbarian because a melee amazon is more of a glass cannon and can't sensibly tank (I usually solo without mercs, and SP offline). When i said primitive I meant the nature of the game, Diablo 2 can be hard but it's not mentally challenging at all, most if not all of its difficulty arises from the fact that many builds are absolutely gear dependent. If you have the right kind of gear it's a walk in the park, otherwise you just die a lot regardless of how well you know the game. No wonder a select few cheesy builds emerged and everyone else got the short end of the stick.

PS: the little stygian puppets in Mephisto's temple could almost one-hit my amazon, not quite so unlucky with barbarian

It's a tie between Beyond Good and Evil and Planescape Torment.

Planescape Torment is one of the best RPGs I've played in term of story (although I can't deny it's lacking in term of gameplay).

Beyond Good and Evil is such a good game I somehow always goes back to it, even though I've finished it at least three times.

Baldur's Gate 1 and 2 are growing on me, though.

Of the game I've got on GoG, I haven't even played them all yet. I played through Eschalon Book I and Fallout 1. Beyond that, I"ve only touched Darklands and Fallout 2. I will one day get back to those. I'd have to say I enjoyed Eschalon Book I more than Fallout 1 because the excessive crits in Fallout 1 got on my nerves. However, in terms of replayability and overall story, Fallout 1 would be better.

Right now I"m playing Anachronox and having a blast. I got it along with Omikron and Risen for $7.75. So lets just say I have plenty of games to play on my "shelf" and yet there're still so many I"d like to try out on GoG. I love GoG.

You are a good purchaser in that everything you get is good but more than that it's loooooooong. I myself am a sucker for throwing down money for 5 hour jobs and I always feel kind of stupid when I finish. :P

Yeah, those little creatures are insanely annoying. Now I only play Diablo II with non melee type characters so I don't have to chase them down...