A selection here:
https://www.sjscycles.co.uk/dynamos-charging-devices/You'll find quite a few others if you hunt around.
The basic decision is whether or not you want a charger with a buffer battery.
Chargers with a buffer:
B+M USB-werk, B+M Luxos U (light), NC-17 Connect Appcon GT
Chargers without:
Sinewave Reactor & Revolution, Supernova/Cinq "The Plug", Igaro D1, B+M e-Werk, Axa Luxx 70+ (light)
If you don't have a buffer, whatever you are powering loses power when you stop. Some devices, like most Garmins, will say "external power lost; continue on battery power?", and turn themselves off if you don't poke the "yes" button quickly enough. Others may just fail to restart charging when you set off again, and most will just silently recommence charging.
I'd recommend playing with what you want to power using a mains USB adapter, and turning the socket off and on at the plug to see what happens, and how annoying you think it may be.
The disadvantage of using a battery is that there's a substantial efficiency hit compared with a non-battery charger, so you may have to ride 30% further to fully charge your phone. If you have a high power requirement, eg for an iPad, this could be a problem.
A buffer also doesn't necessarily avoid the problem of power loss completely. If you are using power faster than you are generating it, maybe when riding slowly up a long hill, the buffer battery will make up the shortfall until it runs flat, at which point the charger stops supplying USB power and starts charging its buffer battery.
If you use a charger with no buffer battery, there's always the possibility of passing the power through a USB power bank that supports pass through charging, such as the Zendure A2. A generic battery pack like the Zendure is quite a lot cheaper than those sold specifically for bike chargers.
A pass-through power bank would have the same efficiency hit as a built in buffer, but the battery is quite a bit bigger so you would need a bigger hill to run it flat, and you've the choice whether to use it or not.
There are also quite a number of comparatively cheap chargers. I would recommend being cautious if you get such a charger, and not leaving it connected to the hub if there's nothing using the power generated. That includes full batteries.
There's a fair record of such chargers dying after failing to cope with either the high voltage that a hub that's not delivering any current, or the heat build up if a basic method of coping with the voltage is used.
I use a B+M eWerk, bought at a time when there wasn't much choice. It keeps a phone, a Kindle, a head torch, and an Anker 6700 power bank topped up with plenty of leeway. The GPS is an Etrex that runs on AA, for which I just carry spares.
