The golden rule at the back is to always have at least 2 good lights. You can't see them once you're riding and if one fails the other keeps you visible and legal.
Round-town bike has a dynamo Smart + Smart R1 (very bright) + Smart superflash (bright and obsolete)
Tourer - used a lot on unlit country roads - dynamo B&M Toplight + 2 Blackburn Mars 4 (very bright)
I used to use all Smart battery lights but the clips have broken on several. Changed to the Mars as a result.
Got them from here.
The Mars lights received plenty of compliments from other riders on the overnight EE ride in August so they seem to work.
fossyant mentions silicone grease on Smarts. I put a smear of silicone grease on the seals of all lights, dynamo system connections, and anything else electrical which might get damp or wet, front and back. I've never had a water problem with any of it.