The two-shot, for sure. I mean, a handshake would be nice because I'd be all "OMG I TOUCHED MY OSHIMEN" and I'd die of happiness, but from what I've heard of these events they push you along REALLY fast; you barely get to say two words to your idol before you have to leave. An autograph would be nice and I REALLY want some autographed merch,but I feel like it'd go too fast as well. A two-shot would be nice because you'd spend more time with them and it would forever be a reminder "LOOK, you did THIS with your oshimen!"
At this point, though, having been to zero events like this, I'd like any of them.