Join Date: Sep 2005
Bikes: Windsor Fens, Giant Seek 0 (2014, Alfine 8 + discs)
Today, LiIon without a doubt. You can HAVE my old NiMH battery.
For tomorrow, none of the above.
Lithium iron phosphate (LiFePO4). They're a bit hard to find now and a little expensive, but I think they'll get more common. They're already the hacker's choice on eBikes and are used by some high end power tools.
Very high capacity, very long life, almost impossible to abuse to death. Not as subject to runaway overheating / outgassing (fires). Also, non-toxic (LiIon are toxic) and eventually may be cheaper.
Work: the 8 hours that separates bike rides.