Best laptop bag for the tube?

[herecomesthsun]

I am looking for a new bag, would like a messenger with a wide strap, that will take a small laptop/ A4 files, be reasonably secure and especially be comfy on my shoulder.

I would consider a backpack, especially if it converts to a messenger bag as needed.

Syle as well as functionality would be good, though I am looking at men's bags too.