I believe the 4x26650 battery packs have the most capacity. At least it seems to be more than my 6x18650 battery pack.
These threads have people that have used both and tell where to buy them. If you're only looking for information about alternative battery packs, you're probably best off from starting at the end of the thread and moving towards the beginning.
Thanks for all your replies. I have been wading through all your links leaftye. This should keep me busy for a while
Anymore experiences with the best battery for this bike light then much appreciated