> most efficient

You don't want that, you want something you can build. A simple one is a
spoked wheel cut from some magnetizable material with low hysterezis (like
soft iron nee mild steel passed through an oven at 800C and cooled
slowly). On this you wind a single wire such that each spoke bears an
identical winding and all are in series. This goes like left-hand winding
on spoke #1, right hand on #2, left on #3 ... The number of spokes will be
even and small, but large enough that you can put a good sized magnet
between two adjacent spokes (i.e. the space between two spoke tips equals
the distance between the poles on your magnet. You can put as many magnets
around the wheel as you have, and reduce the gaps using more bits of
softened iron pole pieces). For 6V, 50mA at 100 rpm, you will need an
awful lot of wire however, even with a 12-pole generator.

Beware that these generators tend to make very high voltage at high rpm (a
few 1000's). 400V is easy with 6 spokes (each winding makes only 66V).
Maybe you can run Nixie tubes off of it ;-).

If you can manage it, arrange for a rotary transformer at 50Hz. I do not
know if one scrapped from a VCR head will work at LF, but you can make a
good rotary transformer from almost any electric brake (inline disc or
bell type). You need two of the same kind and some skill. When done, one
is spinning and the other not. This would make it work even at low rpm (or
zero rpm).

bye,

Peter

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