Just putting out a note that I added a poll feature for this thread, so as well posting which one(s) may have been the easiest for you, you can input it for the poll too. :)
As for myself, all the Gym Leaders were pretty easy. I tend to reach ahead of their levels though, so I don't expect much of a challenge when I do so. Each Pokémon in my team have a different typing and a variety of moves to keep them running against most other types, so that's a main reason as to why too. Though if I had to say any it'd probably have to be Cilan (since it was him I went up against; from picking Oshawott as my starter), for both the reasons of him being the first Gym Leader in Unova and myself having a type advantage when I had went with Pansear for a short while.