You can handle nearly every transportation situation with three pieces of equipment: a nimble city bike, a capable bicycle trailer, and a high-capacity cargo bike. I'm not suggesting everyone can run to the bike shop and buy all three of these wonderful machines at once, but if you set you mind to accumulating them over time you won't be left wanting for a better solution.
Because few of us are always carting a big payload, some of the time you may want to get around town on something lighter and nimbler. A good, lightweight bike with fenders (keeps you clean), a lighting system (keeps you seen), and internal hub gears (neat-o keen!) is the best combination of reliability, versatility, and speed. When you hook up your trailer to your lightweight town bike you magnify your cargo capacity two to ten times over, depending on the trailer.