Best messenger bag brand for motorcycles-waist strap?

I'm looking for a messenger bag with a secure strap system that's used by riders. I'd like something big enough to fit a 15" macbook, textbooks and other stuff. I've looked at chrome, timbuk2, pac designs, mission workshop and seagull bags. I want to know-what do riders recomend to use while carrying a laptop? Tail bags can cause damage, backpacks are uncomfortable and I don't want magnets messing up my machine. I read messenger bags are best. But which one???