I voted for Dragonlance, because it's got the established plotline and suits the established at-least-three-movies standard that Hollywood seems to operate within these days, especially where geek flicks are concerned. The main problem is twofold: it's way too Tolkienesque, and it's way too expensive to do well (without the cachet of the Lord of the Rings name to justify the expense).
I voted for Spelljammer, because I think it would actually be a very entertainingly weird thing to see what they'd make of it. A simple story focusing on the hunt for the Spelljammer, for example, or a condensing of the central storyline of the uneven Cloakmaster Cycle of novels into a single story - basically, concentrate on the quest to find out exactly what the hell the cloak is and what it's for - could be pretty entertaining, and space is cheaper than Middle Earth or Ansalon.
Finally, I voted for Planescape, despite the fact that it would never work. I would actually be pretty interested in an ongoing television series set in Sigil, since the lower expectations of television visuals would mean that they could do without flashy effects and concentrate heavily on set design and visual mood. You could even get away with doing the various planar inhabitants with physical suits and puppets like in The Dark Crystal - because, of course, skeksi are vrocks - Labyrinth, or Farscape. Plus it lets you have a Star Trek-style "location of the week" if you want, simply by running off through a portal, and they can be even weirder without worrying even a tiny bit about scientific accuracy.