There probably is no "best" single-AA flashlight, just the one that best fits your needs. The most technically capable overall in terms of output that I'm familiar with would be the ZebraLight SC51 series and the ThruNite Neutron 1A. The SC51 has a side switch (electronic type, like a mouse button), which some people prefer, while the 1A has a tail switch (reverse clicky with no momentary-on, unlike your Surefire Backup). If you prefer the same type of configuration and switch that you're using now (forward clicky tail switch), then the 4Sevens Quark AA Tactical is one good option, although you'll give up some output with it, especially if you're going to use alkaline cells (not recommended but could be workable with the SC51).
Clearly we need more information. For starters, do you have any preferences with regard to switch type and location? Would your employer pop for lithium AA cells, rechargeable AA cells, or just alkalines? And how about the beam pattern? Do you like the one on your E1B, or would you prefer something different (and in what way)? Tint is yet another consideration. Are you happy with cool white, or would you prefer something warmer in tint (i.e. closer to incandescent flashlights)?