:welcome:
"Best" is rather subjective.
As is "backup". My backup light is an ArcAAA on my keychain. If I were doing something else, my backup would be a 1x123 or 1xAA light...
So, if your main light is a rather large one (Magcharger, Streamlight, etc.), and you like incans... maybe a Streamlight Strion might be of use? SureFire has a plethora of small and powerful incan lights that could be used as backup: 6P, G2, e2e, etc.
Maybe if we had more info on what your primary light is, and more features that you'd like in your backup light (size, material, number/type of batts), we might be able to narrow down our suggestions.
Oh, and :welcome: