Hard to say exactly, but Worldbuilder is a given and Provider probably also works. Were it possible to include fractions of the choices, I might also have chosen a bit of Duelist and Plotmeister, and perhaps even a touch of Master of Ceremonies.
But all of these really overlap quite a lot. A good worldbuilding experience should have enough detail to be nearly as emersive as suggested in Master of Ceremonies - although some occasional bg music (instrumental &/or natural sounds, mostly) and perhaps some slight adjustments to lighting can also help at times. (Who really plays a Halloween game with full lighting?)
I'm no where near as much of a plotmeister as some on this thread have showed themselves to be. My plots do not risk entanglement of myself or the players to the point of mutual confusion, but rather they are the direct result of world building - the details are there and I just take advantage of them. If an area, due to its geography, cultures, etc is prone to bandits, etc, then should the party wander through it they are likely to be set upon. If the area is also ever now and again contested over by two or more adjacent areas, then there is a chance that perhaps one of the bandits is there to destabilize the region in preparation for an invasion (or other form of takeover). Little things like that add up, growing into complex webs of plots. The 'webs' at first are no more complex than a few strands with a couple connections, but after several months of game time the interactions have in and of themselves created a more complex, emersive, and intricate web for the players.
So, really, while I could perhaps have chosen Master of Ceremonies and Plotmeister, I did not, as they are ends that come about from World Building, not initializers in and of themselves, at least for me.
As for the bit of Duelist, World Building requires an attention to detail, and Provider sort of insists upon working towards the interests of the players (and conflict will inevitably be one for one or more of the players). Also, playing a fight poorly weakens the interest and potential enjoyment for the players as much as the it does the GM. If creatures have Int 8, they should not be treated as animals - they have the capacity for ambush, complex tactics, and even strategy. Indeed, even wolves (Int 2) have the capacity for ambush and at least some tactics (Trip, etc). So again Duelist might have been chosen, but it was more there due to World Building and Provider considerations than on its own merits, at least for me.