Gave it a lot of thought and I'm going to have to go with King of Dragon Pass.
But, Vern! Why this semi-adventure game when you own a plethora of exciting, action-packed, 3D games? Well, mostly because there isn't any other game like it. No, it's basic mechanics aren't unique; you are given a tribe of barbarians to manage and every now and then you are called upon to solve random events using text options. I'm sure there are other games that pull off the "text-based management" sim just as well as KoDP.
HOWEVER, I have to give this game a standing ovation because it is the only game I know of that can actually induce culture shock.
In order to actually succeed at playing King of Dragon Pass, you have to actually immerse yourself in the Orlanthi Barbarian culture. You have to think like an Orlanthi. You have to observe their bizarre customs and follow their strange laws. For example, raiding another tribe in the Pass is not a big deal. Sure, your clanmates might provoke the ire of whatever tribe you're raiding but, so long as any murder is declared out in the open and done in the open, no laws will have been broken nor customs violated. On the other hand, if you secretly murder an enemy, this is treated as the cowardly act of a criminal. Worse, if you kill a fellow tribesman for any reason then you will be branded a kinslayer and you will have just invited Chaos into your settlement. Note that this is magical Chaos. As in, terrifying beast monsters will burst out of your cow herds and start trying to kill you.
I could go on about the many different examples of how much culture one has to absorb in order to play but it would take all day. Suffice to say that this game has no peers. It is wholly unique.