I am not sure what you mean by lowest maintenance fixer, but since I haven't tried them all I couldn't really answer anyway. However, I seem to remember Kodak has a liquid that would probably be the easiest to mix and store if that is what you mean.. I always used their powder and it wasn't any hassle to me.
I always reuse fixer solutions until I think they are at a point of getting exhausted, on a conservative scale. Then I it out and mix another batch. If I wasn't sure a few times I would develop the leader and test it in the fixer. If it went too much over five minutes, out that solution went. As I said, I was always a little conservative.
Hope that helps.