I like the theory they had in the TV series Red Dwarf,
first there was the big bang, everything including time rushes out from a single point, eventually it runs out of momentum and starts going backwards, time and matter eventually end up back at that same point again and .. pop out they go again
The Big Crunch - expansion of the universe slows due to gravity, stops and then it all collapses back to a singularity. Requires our universe to be closed, and from what we have/can measure so far this isn't the case.
This is one of my favourite ideas though, given that the big bang is a natural process (keep it scientific please) then it must be able to happen more than once. An oscillating universe where one dies and another is reborn a possible infinite number of times is elegant to me
Still doesn't meet the requirement for the first big bang. Also gives the potential for an infinite number of universes isolated from our own (the natural process part).
The other options where the universe is open or flat are a little depressing ;p
