Ok lets see

Ultimate-well I had KoTR and it only took me one battle to get it to Cosmo Canyon,so umm no...
Ruby-Once you have KoTR W-Summon and Mime he is so easy
Emerald-Annoying but easily beatable if you know how.

I am surprised that noone mentioned Diamond weapon!
He was the hardest for me mainly because I wasnt pumped on steroids(KoTR/mime gasp :/ ) nor did I have ultra 1337 1hit kill limit breaks(omnislash gasps ://) and I was less than level 30.
I beat it in 1st try though,just barely