Hardest Elite 4 would probably be the Sinnoh region's. They were brutally beating me, especially starting with Flint since I had no Fire Pokemon. Cynthia was pretty challenging as well, but not as much as Steven Stone.
I guess sinnohs but ONLY because I think they outleveled me by like 15 levels while other games they would only outlevel me like 5 levels.
Really any of them are easy with spending some time to train your Pokemon.