Finding lights that can run brightly all night is no longer the problem it once was. Now, very bright lights are commonplace and the main concern for cyclists is using lights that won’t dazzle other road users. Unfortunately being legal is also a bit of a problem, because most of the best lights on the market are not, in fact, compliant with the national law. To be legal you’d have to also fit, and use front and rear, one of the relatively small number of lights that is.
At the rear, for visibility a multi-led battery light mounted correctly and used in constant mode (not flashing) is the best solution. ‘Mounted correctly’ means solidly on the bike frame or carrier, not obscured by luggage or clothing, and pointed directly backwards, parallel with the road surface. These lights are very reliable and have a runtime measured in days, are highly visible from a distance, but will not dazzle a cyclist riding close behind.
If you insist on using a flashing light, then be prepared to ride on your own.
At the front, a similar style of light is the best solution for visibility to oncoming traffic. However randonnées often use unlit roads so for serious night-riders there is also a need for a beam-throwing front light with an extended runtime. Modern LED lights can be very bright and it is important to mount beam-throwing lights in a way that won’t dazzle oncoming traffic, and for this reason a separate visibility light is still advisable.
On the subject of visibility, reflective ankle bands are a lightweight and effective complement to any lighting setup, as is reflective tape strategically applied to the bike frame.
As a night-rider, since most lights even at lower price points are easily bright enough, the choice is more about reliability, runtime and usability (eg small switches and thick gloves). Reliability and runtime are both improved if you opt for two lights rather than one - many modern lights are torch-like in shape so don't take up much room on the bars, so consider fitting 2 50euro lights rather than one 100euro one.
Most lights now offer multiple brightness levels and on lit roads or riding uphill you don't need the highest setting. This goes a long way to solving runtime problems. If you ride 200s and never expect to go beyond a summer 300, there are lots of small, lightweight and inexpensive lights that will serve you well, or at a higher price point small rechargeable lights designed for commuters can be ideal. Even on a midsummer 600, allowing for dawn and dusk, some lit roads and some time in controls, you are unlikely to need your ‘serious’ light to run for more than 4 hours.
In spring and late summer or even later in the year, it’s a different matter entirely. And on PBP (which has some very dark unlit unpainted roads) you may need more than 20 hours of runtime, which is challenging even with today’s technology.
Generator lights are popular with experienced randonneurs who may ride long events ‘out of season’ or tackle multi-night events. Hub dynamos get very high approval and are considered reliable, bright enough, ‘always there’ and of course with infinite runtime. Carry a small torch as well, for roadside repairs and as emergency backup.
Battery front lights can be less expensive and more flexible - you can swap them from bike to bike, and you only have to carry as much lighting as you think you will need. They must have runtime matched to your needs, be reliable, and bright but not dazzling.
Types that rely on rechargeable batteries may be preferred by frequent night-riders, such as commuters and hardened out-of-season randonneurs. However for the very long multi-night events such as PBP rechargeable batteries present problems (one reason for the popularity of generators). At least one battery light should be capable of running off primary cells easily obtainable along the route. Lights that consist of a head and separate battery pack work well, because the standard rechargeable pack can be replaced if needs be, by a made-up clip of primaries. However these two-piece lights vary hugely in build quality, beam shape and price - from £20 on Ebay to the sky’s the limit from high-end manufacturers. Even the cheapest ones offer lots of brightness and lots of runtime - at least until it rains! Many have a very wide beam more suited to off-road use than on-road where traffic will be dazzled - but the better ones with a more ‘spot’ beam are among the best battery lights you can buy - if you can live with the clutter of a separate battery pack.
Helmet- or head-mounted lights can be a useful supplement to your main lighting rig, but are frequently found to be worse than useless in conditions of high humidity (all too common!) because the beam just reflects back into your eyes. A small finger-mounted light may be more practical, for checking route sheets and signposts, or rummaging in the saddlebag.
The serious night rider will always carry spare lights, for both front and rear. Reliability is key at the rear, and water ingress the main enemy, but almost all rear lights can be greatly improved by application of PVC tape to vulnerable areas. Front lights often run too hot to be taped like this, and so will be less reliable in wet conditions - another reason to carry a spare for the front.