Hard to say for sure what's the toughest as I haven't abused my lights enough to actually break any. I would assume SF lights - particularly SF LED lights - ought to be tough based on their reputation and cost. Otherwise, I would GUESS design features that would make a light tough include: potted electronics/circuitry, LED rather than hotwire, polycarbonate - not glass - lenses, mechanical support so the batteries don't rest on a delicate part (like the PCB), high quality switches (or no switches), and good engineering design (again, by reputation).
In general, I think smaller lights will suffer lower stresses for the same shocks than would larger lights, all else being equal. It may well be that the $1 "toughtest LED light" from Countycomm could survive a drop test that would kill any other flashlight made, so it would help to know what size of light you're interested in.
Excluding the very small keychain lights, the most overdesigned small lights appear to be the old Arc AA/AAAs, the old CMG Infinities/Ultras, and the similar Peak lights.
Tektite LS Splashlight ($20 at BatteryStation) looked pretty small and robust for a 1xCR123 light (and SF's, yada yada..).
I think the 3xAA/AAA battery carriers are a liability for EXTREME shocks (but otherwise work OK). Past that, I really can't guess as most of the 2xAA and 2xCR123 lights look pretty robust. I'd have to start dropping them (spin stabilized so they always land lens first... /ubbthreads/images/graemlins/wink.gif) onto concrete from measured heights to really figure out which one really holds up better.
Apologies for being long winded...just killing time on a quiet afternoon... /ubbthreads/images/graemlins/smile.gif.