Excluding the Sinnoh League (for obvious reasons).
I think the Unova League would have been way easier had Ash brought back his Hoenn/Sinnoh team. The fact that he lost to a recently evolved Lucario left a lot to be desired.
Kalos League wouldn't have been so easy because of Alain (unless...
The Pokemon anime always has a way to Link the current generation to the next generation. It's been like that since the very first series:
Gen 1 to Gen 2:
All of them appeared before Gen 2 started.
Gen 2 to Gen 3:
-Blaziken in the Johto League finals...