That depends on the subject, location, time and result I want to get.
For instance, when I want to make big prints and I can plan everything out exactly as I want to, when I know the location and how to get there - I take my 8x10" camera with 240mm and 360m lens and 3-4 film holders.
If I need to be more flexible I get my 35mm kit, that's either a Canon 5D with various lenses ranging from 24-150mm for colour photography.
When I'm out of the field I tend to bring the A-1 with the 50mm, it's rugged, lightweight, reliable and can live of a battery for months.