Looking for flash advice (powerful flash)

Buying a flash can be a bit frustrating. I'm looking for a flash that's powerful enough to light medium size groups of people, but comparing the power ratings is nearly impossible. They each use a different set of measurements to define the unit's power, so direct comparisons are difficult.

Can anyone offer a recommendation for a powerful shoe-mount flash for a m4/3 camera? Sometimes the built-in flash just doesn't cut it.

FWIW, I doubt I'll ever use off camera.

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