I prefer to have several rough plot hooks available, each with fully statted monsters & NPCs, maps and treasure. If the players don't use them, fine. I can always recycle the parts. I know who lives where, what their issues are and how they'll react to the party. This is freeform enough that I can be prepared for the players to do just about anything. However, it takes lots of time to prepare for this kind of game.
Sometimes I get the wacky idea that running a published adventure will save me time. But my players have never...NEVER...followed the path set forth in a published adventure. So I end up doing it all myself anyway, but on the fly. Unsatisfying for all involved, since I just don't DM all that well by the seat of my pants.
The one exception is some of the Necromancer modules. They're really more like mini campaign settings than actual linear adventures, so they're akin to the sort of thing I'd put together myself.