I've been happy with this

https://www.performancebike.com/shop...dlight-40-5370

Long run time, small enough to keep on the bike, & bright enough to see the road a bit if your main light gives out. Rigged it to sit under the garmin mount.

Also like these

https://www.performancebike.com/shop...-combo-40-6306

Run pretty much all year on a set of coin batteries, but can handle only a moderate amount of rain.