I've run Planescape in D&D 2e, 3.Xe, and 4e (and actually will be running some Planescape soon). The setting as written works best with 2e since that was the system for which it was designed with all the tropes and assumptions of that game in mind. If you're just using the flavor, you can use any game though I still believe there is something lost in the translation. I'm still not going back to 2e, however. I don't recommend running any published setting unless you and every single person playing it is at least somewhat familiar with it. If that's not your setup, then I'd recommend just coming up with your own homebrew setting inspired by Planescape on an improvisational basis with the help of your players.
