Batteries are a major hassle for photographers - particularly photojournalists, who must rely on their batteries staying both able and not corroding gear despite being in scalding-hot car interiors in summer and freezing car interiors in winter.  Fortunately, some do work fine every time.
   In the “AA” alkaline batteries flash units need, my strong preference is Ray-O-Vac; they don’t corrode - and stay powerful for six months unused in a flash left in a car.  I’ve also never had problems with Duracell.
   I’ve had corrosion troubles with Eveready alkalines - fortunately never in photo use, as I simply won’t use them in that!
   My choice of where to get batteries is Lowe’s - as they sell them cheap in discount volume packs for building tradesmen, ensuring high turnover of those for sale and thus fresh ones.
   For rechargeables, I’ve never had trouble with either Eveready or Promaster Ni-MH batteries for powering digicams. 
   But - for proprietary battery packs for dSLRs - my choice is either Green Extreme or Promaster lithium-ion rechargeables.  Either is much less expensive than Canon’s - but works just as well.  Adorama sells Green Extreme inexpensively.