The title is true, If you think about it. Donkey Kong Jr. doesn't even have girders, DK3's girders are not that tall. You have to navigate many more girders per man in Donkey Kong than either of the other kongs. DK is certainly the hardest of the 3 with regard to girders traversed per man, which in my opinion makes it the hardest. Besides, this IS a free country and I'm entitled to my own beliefs
Now, lets consider a more empirical approach; how about a ratio between Man for "how high can you get". Donkey Kong requires the most vertical climb per man of the entire series.
Donkey Kong allow's for jumpman to gain 150M in height for all levels cleared from level 5+
So...
17 (levels 5+) * 150M = 2550M
2550M + 50M(level 1) + 75M(level 2) + 100M (level 3) + 125M (level 4) = 2900M = 1.802 Miles.
That is much higher than you can get in either Donkey Kong Jr. or Donkey Kong 3, so It's the Hardest with regard to change in elevation per man. I think that D2K might have a "potential" to de-throne DK as the hardest game with regard to elevation change.