I would like to go with Chris Hadfield. He has quite a lot of experience, so that would be handy if we came across any problems. He has done lots of experiments in space, which I would like him to teach me. He also plays guitar and sings very well, so we would have plenty of entertainment. He seems like a really nice guy too.
I’d go to space with Darth Vader (I’m assuming he’s considered an astronaut) and then have a very very long chat with him about his life choices :)) If you were looking for a real life astronaut, not sure who I’d choose – don’t really know any of them well enough to decide.