David Tennant
He seems like a sweet and amusing guy, and if I'm stuck in a lift, I am going to need someone to distract me. His accent alone would probably do the job. Besides, opportunities for Whoniverse jokes and quoting Shakespeare are endless.

Neil Gaiman
He'd definitely help pass the time, plus I could take advantage of the situation to whine about my uni degree and the literary types I have to put up with. I'm sure he'd understand. Also, he'd Twitter about it which would be highly amusing.

I think two people would be the most I'd be able to handle (and might even be pushing it) before I lose my mind and start clawing out my eyes or something similarly pleasant. It's something I really hope will never happen to me.