Liza and Tate were always the hardest gym leaders to me. Obviously, in ORAS, no gym leader was really hard, but when I used to play sapphire as a kid, they were so difficult, and I think it took me about eight or nine times to finally defeat them. I didn't one single bit about double battles. Annoying twins being annoying.
The most frustrating attack though, must've been the Calm mind. The maximise it, and I technically had no escape at all. I tried using all the pokemon I caught, but still no result. I finally managed to do it, after a long time of training and working hard. I think I made my pokemon five levels higher just to beat them.