There was a stiffness test done a while back, someone moight be able to find the results. The problem is it's dated now and a few of the stems have changed design or been discontinued. From memory, the PRO Vibe track stem was top dog and the 3T alloy stems with the wider front plate bolt spacing (can still be found as a MTB stem) rolled in after.
NOTE: do not discount the part your steerer, fork and bars have to play in how flexy your front end feels. Decent quality stems would show negligible flex compared to those other parts in the chain