If it is even possible to pick one!
I think if I had to pick one, it would be Escape From Dino Island - a Jurassic Park inspired game where you are dealing with dinosaurs on an island! It has some great world and adventure building mechanics to kick off the game, and I think one of its best features is a mechanic called "Tell a Story". When you take an action when you're not in danger, you pick from a list of story prompts your character would tell, and tell a story as you complete the action. It's a really nice cinematic action/downtime split built directly into the mechanics, and really gives you a helping hand with structure. It's probably my favourite PbtA game to introduce new players to the system!
Next would be:
Girl Underground - inspired by Alice in Wonderland, and Labyrinth. You collectively play a girl and her fantastic companions, figuring out how you interpret or reject the expectations and rules imposed on young girls. A beautiful and thought-provoking game!
Monster of the Week - think Buffy the Vampire Slayer, Supernatural, X-Files, Fringe etc. Modern monster hunting and mystery solving!
