Unova is objectively the easiest, because you can face them any order, and each of the E4 only have 4 Pokemon instead of 5 first time you face them. Johto E4 may have lower levels, but the levelling curve in Gen II and HGSS is pretty low.
Really? I swept them with just four level 40-42 Pokemon!Gen 2 has been the easiest for me. All it takes is a team of level 45-50 Pokemon to beat the Elite 4 and the Champion.