Which shout does the most damage?

Nothing beats Unrelenting Force level 3 if you can push the NPC down a cliff/stairs etc... Instant kill ftw

Appart from that, I guess Call Storm is the most damaging shout you can get, all other options are kinda lacking. It's hard to evaluate the effect of summon shouts though.

Marked for Death does 3/sec for 60 seconds and -75 Armor as far as I know. Fire Breath does 90 Damage, the Ice thing does 2/sec so 120 in 60 Seconds FREEZE, meaning if you only autohit that target while frozen this should be the most damaging. The other ice spell does 90 dot damage, and slows wich can be really nice for multible targets...

Storm Call has the highest damage value over its duration, that or Call Odhaving . The base damage for Unrelenting Force is actually something like 10 points, although it does non-listed damage based upon physics. Odhaving could theaoreticly match it by doing the "lift and drop" attack to also inflict physics damage. Note that Storm Call has no IFF and cannot work indoors, greatly limiting its use.