A well-worthy contender for greatest game of all time.
The first thing you see in Planescape, is your character dead, on his way to the mortuary. And then you wake up, you've got no memory of who you are, you can't die (if you die in in-game combat, you just pop up revived in a safe place), your body is covered with scars and messages to yourself and it's clear that other people in the world recognise you but they've all got their own secrets and aren't honest with you, and you seem to have been a powerful person in previous lives whose done a lot of things.
To put it simply, the writing is amazing. What's the game about? It's about finding out why you keep dying and putting an end to it. Or finding a way to die permanently. Or understanding the question 'what can change a man?'. Or discovering who you really are, not just what you've been but how a man can truly know himself.
... It's not just this game has a lot of themes, but how you play can change the very fundamental of what this story is about and all these different themes have echoes and reminders that you can have throughout play and even the very climax of the game can reflect this.
And the written dialogue allows so much, there are cursed items, items you'll have to explore, read, learn about, take with until you finally understand whats going on. You can dig clues out of zombies ...
The most fun in questing, is finding mainly dialogue based solutions, learning things about characters and the world, you can just fight your way through everything, but the king stat is really wisdom and the other speech stats. I spent a happy hour in one unique brotherly, with 9 people, running from one to the other, uncovering secrets, solving problems, sharing life goals. There are lots of moments in this game where theres dialogue not focused on advancing, but just giving you the opportunity to say what your goal is.
Saying that, it's a contender, but probably isn't the greatest game. Combat is stale, formation based, but almost no ranged weapons which makes it a bit pointless. And because talking is so much fun, it can be easy to keep the PC underpowered in a fight and there are sections of the game where you probably will have to fight.
And the licensed setting holds it back a bit, there are things they can't reveal and people you don't interact with as much as thought. Yet all in all a completely fantastic game