The best type of shower to use with a combi boiler is an electric shower or mixer shower.
An electric shower provides instantaneous hot-water, retrieving cold water from the mains and distributing it around the system’s internal heating unit.
A mixer shower connects to both the hot and cold-water supplies, adjusting water flow and temperature using a shower valve or the taps.
You should avoid power showers with combi boilers. Power showers have an internal pump and this makes them incompatible with combi boilers due to the additional force they create potentially leading to excessive pressure.