@berlinonaut what would you say the best option today for dynamo and light for Brompton? The best light which delivers best and brightest beam? Only front light. For dynamo what would you say the best option in terms of minimal drag resistance when light is off and lower drag when used?
The best hub dynamo is no doubt the
SON. The best light is a bit a matter of taste. In my opinion it is still the
Edelux, though the
Bumm IQ-X is a bit brighter and the
Supernova E3-pro claims to be in the same range. I do know all three in practice on the Brompton and clearly prefer the Edelux - in my eyes it has the best beam of the three. Clearly better than the Supernova. The IQ-X can - when sorrowfully adjusted - be as good (or depending on your taste maybe even a bit better) but adjustment of the IQ-X is a pita plus it is mechanically prone to failures (mine died after 1,5 years and I am not alone while my Edeluxes last since I bought them many years ago) plus the IQ-X has a switch that is completely useless. I also prefer the light color of the Edelux over the very cold one of the IQ X. In opposite to the IQ X the Exelux clearly has the best usability and the best mechanical quality - do run currently three, one first generation one and two second generation ones.
Be aware that you can buy the Edelux in a normal version, intended for the position near the front brake, but also in a hanging version that you can mount under the bars if you prefer that.
If you leave the range of lights compatible to German StVo Supernova offers the
E3 triple3 which should be recognizably brighter than the legal ones. I do not have personal experience with it but know people that are convinced by it. There are also the k-lite
Bikepacker Ultra and
Bikepacker Pro from Australia which claim to be the brightest dynamo bike light (intended for MTB-use). Again, no personal experience.