Like everyone else here has stated, there are multitudes of games in terms of replayability.
Considering this, here's my attempt at some GOG Addictions:
{Note: These are my personal favorites, recommendations based on your preferences are to follow in list form}
Baldur's Gate II:
Let it be said that there is a good reason why Bioware have not made a direct sequel in the Baldur's Gate series. Well for one, this game has a great conclusion with the Throne of Bhaal expansion. Two, the amount of effort put into this game is staggering, considering the amounts of voicetracks, the music, the artwork, the plot and the overall gameplay. This is one of my favorite RPGs of all time. I have clocked in 180 hours during one playthrough. Or was it 800 days of game-time? I don't remember. All I know is that I have fond memories of playing this game in high school and how I was the coolest kid on my block for having this game in the 5-Disc package it came in.
HOMM 2 & 3:
Everyone else has stated how that these games allowed for infinite possibilities in terms of replayability. As great as the campaigns were for these games (from a turn-based strategy standpoint), I had the most fun back in the day with the local multiplayer and just setting up random scenarios and trying not to get my ass handed to me by the CPU. I will be honest, I was an idiot when it came to this day and would have me vs 7 computer players... D'OH!
The Last Express:
I could say so much about this game and how to this day I still love to play it just to see what different choices I can do and where I end up.
If you are unsure about this game, read this well-written article about it (complete with a dash of humour):
The Last Express @ Crapshoot Arcanum:
Fallout with elves during the industrial revolution. This game is an epic example of steampunk with a unique twist. I know that there are bugs in this game, but honestly I was so drawn into the setting, the plot, the music and the gameplay that I consistently find myself going back to the world of Arcanum. The game can be challenging but also rewarding.
Plus, who doesn't love having sexual encounters with sheep?
*BAH!*
The Longest Journey:
I have every puzzle memorized and every plot twist as well, yet this adventure game is something that I find myself going back to when I need to just unwind after a long day. Honestly, this game is an example of how a point and click adventure game should be. Yes, there are the treasured Lucasarts games of old, but this one is a treasure as well.
Besides, Crow is by far one of my favorite characters in any game.
Planescape Torment:
What is there to be said? A great RPG, with a storyline that resonated with so many due to its creativity and originality. Great voice actors, great storylines and characters that you actually gave a damn about. Plus, Morte is my hero. Even if he tends to hump anything that has... actually, Morte humps anything.
Beyond Good and Evil:
Some might say that there is no reason to revisit this game after you beat it. I say, nay nay! Any game that is this beautiful in terms of its artwork, engaging characters, fun gameplay, astounding soundtrack and awesome setting; is always worth another playthrough... or 10... I am not addicted... I am not addicted...
So those are some of my choices.
However, considering your preferences.
1) HOMM 3
2) Space Rangers
3) Arcanum
4) Baldur's Gate II
5) Settlers II
6) Tropico
7) Age of Wonders Series
Enjoy! And let us know what games you end up getting. :)