Heat Death, depressing as it is, seems to be most likely. However, I also believe that it's quite possible that another Big Bang will occur after the end. It did happen once seemingly out of nowhere, with seemingly no reason. That, and I like the idea of a never-ending, cycle of creation and destruction. Keeps things interesting. Especially when you begin to wonder, if the universe rebirths after death, just which time around are WE on?