It depends upon what I'm playing and where I'm playing it.
I tend to host a lot, so that usually eliminates the issue.
When I'm not hosting, I generally use a particular green backpack.
However, in recent months, I've been helping our DM with minis & such, so I have a rolling suitcase that I've been using, and an expanding modular tube for the megamat.
On the flipside- had I not been expecting to share some of my books, I would have appeared at the Dallas Gameday with just my mini, my dice, and my PDA with my PC file on it...maybe a PHB or 2.