The best DMs are the ones that can adapt and come up with stuff on the fly. The only guaranteed thing is that the players are idiots and will try and punch a NPC in the tavern on day 1.... throwing your well planned story out the window.
Adapt and come up with stuff on the fly? If by that you mean TPK when so much as a single dice roll diverges from my meticulously crafted grand plan, then I may just be the single greatest DM of all time.