The 28mm regular ones (not Plus) with the slight tread, flat protection and reflective sidewalls. My bike store had more slick 28mm marathons but I wanted something that could take the occasional fire road and I was told that the knobby option was better for flat resistance. Obviously they're not as good as something fatter when the going gets rough, but they definitely do the trick and it's actually sort of fun when you to have to be extra careful how your line goes. And they work great on the road.

I haven't flatted in eight months on the bike of mine that has Marathons, which is how long the Marathons have been on that bike.