The Fenix L2D-CD/Q5 has been the brightest 2xAA light available for some time now, and it still is today.
It's almost a "perfect" light because it's exactly the same size as a MagliteAA, puts out an amazing amount of bright light, but also has other brightness selections to where you can use it at a lower power level and make the batteries last over 50 hours with still plenty of light to get the job done.
As far as batteries, it works great on Eneloops and other NimH rechargeables, but it also eats Alkaline and Lithium primaries so you have plenty of options for when the house lights go out and you can't recharge batteries anymore.
The LED itself will NEVER have to be replaced because it will never fail, so replacebility isn't an issue at all. And while you can upgrade many lights with newer emitters as they are developed, the L2D is built as a "unit" so even if you could change out the LED later, I wouldn't. I'd just buy a "new and improved" version when it ever comes out and keep the original as STILL being a helluva light to have on hand.
Seriously, I have a handful of lights which include a DBS, CL1H, A9s, AE Xenide 25W, converted Maglites, D-Mini, E1L (and many others), but if I see a bright, mushroom shaped flash in the distance some night and have to immediately head out of town, the only light I'd take is the L2D-CE/Q5 and a package of AA primary batteries.
It's that good.