Voted for Planescape, just because it can be the most chaotic, varied, whimsical and dynamic. With as dark or lighthearted a tone as none other setting.
And it can encompass as much of the multiverse as you want. Think old school Bioware-style RPG: like commander Shepard jumping from planet to planet for various quests, but this time it’s the planes. Possibilities are endless with Planescape, same for Spelljammer ofc, but I’m not that familiar with it.
