I hate D&D. Rather, I hate the samey nature of D&D. I hate the implied setting, color-coded dragons being the norm, the brain suckers and the eyeball-fucks. I hate every nerd who tells me that I can't do XYZ in their game "cuz." I hate goblins, elves, orcs, and everything pillaged from Tolkien's tomb.
Why am I bringing this up? Because-Because Planescape lives rent-free in my head. Published originally for 2e AD&D, I can't think of a worse combination of setting + system out there. It's nonsense. You play in a not-industrial fantasy city on the inside of a giant car tire at the top of an infinite spire in an infinite world of pure-neutrality. How do we know this? We just do, berk, don't ask questions. In this city, locals speak a dumb cockney slang with a plethora of added gobbledygook.

What makes the city so special? It's got a bunch of portals to anywhere you wanna go. "CUT THE LINES OF ADVENTURE AND JUST PORTAL YOUR WAY TO THE FUN STUFF," the setting touts. What constitutes a portal? Literal portals. Doorways, gateways, windows, open furnaces, do you surround a place and act as an entry way into another place? Well sir and/or ma'am, you might just be a portal! But the portals need keys... What's a key? Thoughts, items, feelings, actions; literally anything that can be presented to your gaping entryway of choice may be the key!
"But where do my players go when they take a portal," I hear your cries. Any/Everywhere! Namely, other Planes. What are Planes? Well, a bunch of infinite worlds that correlate to the alignment chart that everyone knows and loves (with some extra bits sprinkled through for flavor). Want to LARP in your RPG and reenact A Paladin in Hell? That's just a portal away!

"Dang! That sounds awesome! There must have been a lot of content put out for this setting to support all of these infinities!" Woahwoah, heynow. Let's not get excited. Were it created today and by some homies with OSR sensibilities, there may be some mechanical edge (random tables galore) to compliment the implication of countless countlessnessses... but sadly, none-to-be-seen. Like much of the other content of the 90s, it's got a lot of setting fluff and flavor that is a fun read but poor for gaming. This aforementioned tire-city (called Sigil (pronounced Si-GILL by its locals)) is iconic in its own right and the sexiest and most developed part of the setting.
I attribute this to White Wolf's dominance of the RPG-space in the 90s. I also look at Planescape as not only TSR's natural reaction to the EDGE-filled climate of the time but also their reaction to the faction-based settings of White Wolf's games.

If Sigil is Planescape's sexiest aspect, the sexiest aspect of Sigil is its many factions. These are all expected to be joined/playable by the players and handle the municipal functions of Sigil. Join the Xaos-exterminating city guard with your buddy the knock-off Judge Dredd and tolerate the book-worm nerd who gets off to learning about laws. The possibilities are endless (as long as your definition of endless is around 15.) But don't worry, you're not some square who sits around doing paperwork all day. With the bulk of Planescape taking place in D&D's Planes of Belief, belief is power and that's the factions' main priority. Spreading your philosophy far and wide in the infinite infinities. Or at least, that's one option if your players give a shit about over-the-top goofy twists on real world philosophies in this over-the-top goofy city.

Not only does it make no sense from a world-building point of view (however intentionally) but what are you meant to do in the setting? Fight a spiritual war against those who think differently than you? Sure, what rules are we screwing with again? Basic trad-fantasy might & magic sword-and-sorcery-derivative sameness? Aaaaand you lost me. Beyond faction-oriented twists, nothing mechanical was added to the mix. Welcome to Sigil, where everything is the same but with a fresh coat of paint on it. At least Ravenloft had the balls to squirt out a horror/madness system.
But fuck me if I don't love huffing paint.
I look at the 2e setting as a "lie of infinity." I'm immature for thinking that a game could support such a wild and crazy idea in the long run but TSR's also kind of dicks for not changing things up beyond a few added races and creating the factions that may or may not appeal to your players.
How about we fix it then!
Maybe. When I had my first blush with the setting years ago, I converted the factions to 5e. I'm not interested in running established settings anymore but I do encourage you to maybe hunt down some PDFs and look at some pretty pictures and read some funny words.
That all being said, I'm not alone in this. There are many Planescape-inspired works out there, from other games to likely other settings and other blogs that have paid reference to it. The greatest example of "this could be so much more" is none other than its CRPG; Planescape: Torment. The mechanics of 2e and overdone implied setting of D&D take a backseat for the excellent story and setting exploration of the game, there effectively being little to no "D&D-isms" present in the game and best-of-all: inaccurate depictions of established lore. Play it.
The Planescape fansite. Full of early internet cheese and original Planescape content and fiction.
My aforementioned 5e conversion. I won't be editing it going forward, let me know if it finds you well.
A late homie named James O' Rance converted the setting to White Wolf's Mage: the Ascension. I love Mage and while I haven't playtested this combo myself, it's worth considering if you're thinking about running Planescape.