Tapu Koko was the last Tapu I caught. I'd say he was tied with Tapu Bulu in terms of difficulty to catch, but traversing the desert definitely made Tapu Bulu take longer.
I had 25 Dusk Balls (I was playing at night), and I went through all of them and had to reset twice on Tapu Lele. Red health and paralyzed the entire time. Tapu Fini's Aqua Ring was incredibly annoying, and I accidentally KO-ed it twice by being impatient about attacking it to put it back into red health, and I used up my 25 Dusk Balls once. I would say Tapu Lele (the first one I caught) gave me the most problems.