Gravity. It's the simplest and most effective method. Any other method
(electric, venturi, pumped) are usually only used to get around a lack
of head problem, or where piping is a /major/ hassle.

Yes, but to get a reasonable head from a gravity system, you'd need a 10
storey house. OK, a gravity system can be made to get reasonable flow rate
to a bath. However, a decent shower needs pressure, not just flow rate.

Christian.