There is only one answer to this question.........
(Drum Roll).....................
It's SPLASH !!!!!!!!!
Here's what splash does :
Doesn't do ANYTHING.
Source
However, Frustation and Return with highest possible and lowest possible happiness respectively, as UberPwnage said, are the weakest TM's learnt.
The weakest HM(s) undoubtedly is Cut, but Flash is close too.