Bucket Bag. Used them for years contracting. Paired with a small clip on tool pouch and parts pouch you have what you need right there for 90% of all projects... unless you are one of those people who likes to run up the clock walking back to the truck fifty times instead of actually thinking through the job before leaving the truck the first time. The pouches are for running up a ladder.
Hint. Do not waste your money on hardware store buckets. They are crap that won't last. Old machine oil buckets and coolant buckets last much longer. Drywall mud buckets don't last either, but some paint buckets are ok.
Alternatively if its going to be a prolonged project a tool cart is useful. You just have to be disciplined enough to empty the tool cart when you are done.