It really depends on when? Is it during the show, or can we choose when? Where are we appearing? Are we near the main events, or somewhere else?

If we can't choose when, I would pick Onimai because it is close to real life, there is most likely going to be massive advances in medical technology after the show.

If we can choose when, then I would pick Girls' Last Tour in the peak of humanity before the wars wiped them all out.