I travel with a Mamiya 6, 50mm, 75mm and 150mm. I use a handful of filters and a gossen luna f meter. The whole thing fits into a sling style courier bag that has extra padding from a cannibalized laptop case. Included in the bag are around 20 rolls of film.

When I travel most of my shots are street and architecture. The 6x6 format w/o pc lenses is not the best choice for architecture (possibly the worst) though.

The must have lens for me is the 50.
The must have camera is a RF (light, hand holdable, etc).