Any single AA torch (Edit, or 2 AA, even a Maglite LED, efficient but horrible choice of LED) will be using a boost driver. Lots of them out there, but nobody tests overall efficiency in reviews, the nearest you might get is comparing run-times, factoring in output lumens, with who knows which cell, then treating it all with a good pinch of salt.
I still like my Olight S15 which has a boost-buck driver that can run on one AA NimH (1.2V), or 2 AAs with an extension tube, sadly no longer made, or a 14500 (4.2V or more). But no point in using 14500, as I have previously explained. It lasts very well with an Eneloop.
Generally Olight make very good drivers, but they are not usually budget priced, except when on offer.
I also like my Nitecore MT22A, it’s a 2AA side-by-side design, using a boost driver, works very well. It is now my backpacking torch, together with a Mammut headlamp running off 1 AA. Not glamorous, but these work. And are powered by AA, rather important to me.
Edit: and they are very light, being made of good quality engineering plastics rather than dumb cylindrical bits of aluminium alloy, like almost everything churned off a CNC machine, where tooling costs are zero, and work can be shopped out to the lowest bidder, and copied.
18650 torches would usually need a buck driver (except for the ones with high voltage LEDs). The only two I have at the moment are a Thorfire C8S, which can run on one 18650, two CR123 or 18350s or 16340s. It seems pretty efficient too, far ahead of e.g. my BLF A6. And a Nitecore EC4S (Cree XHP50), which runs off two or four cells of similar variety, likewise far more efficient than the A6.
Edit: the Nitecore EC4s is also an example of an un-copyable lightweight design, being moulded from magnesium alloy. The tooling for these ultra-light torches must cost a fortune, surprising they are so affordable.
Many P60 drop-ins use buck drivers to cope with many combinations of cells, Kaidomain is probably the best place to look for these, and their hosts. Probably sneered at by BLF as being old fashioned, but they still work well, and make for a very flexible modular system.