Most Surprising Game
This is such a vague and nebulous question.
Is it referring to the general RPG found most surprising, or is it maybe referring to a specific instance of a gaming session that has been surprising.
If it comes down to a specific gaming product, I'm not going to go with something particularly new here. Instead, I'm going to go with something that really expanded the way I thought about games. That would have to be the game "A Penny for My Thoughts", as a minimalist game it was something that I had dismissed, but once I actually saw it in play there were things that really became clear between the lines. There was a crystalizing moment that made me think that all of the systems and minutiae I'd encountered before that point were basically dross that could be thrown away.
If it comes down to specific events, I'd have to say tat it can be surprising to run the same game ten times over at a convention. The same starting point, the same characters, but things can go in dramatically different directions. A specific case in point was (and I hate to advertise my own stuff), the times I've run FUBAR at a conventions and seen dark drama emerge in one session, followed by slapstick comedy in the next. It all depends on specific choices made by players in the group and how other players riff on those decisions.