I think all Generations have varying difficulties depending on the Pokemon you choose...
However in Gen V they just hand you a Lucky Egg which really makes training incredibly easy. Theres also Doctors and Nurses, vending machines and what not around routes making it easy to heal.
In my view of the most difficult to easiest would be:
HG/SS -> G/S/C -> FR/LG -> R/S/E -> R/B/Y -> D/P/Pt -> B/W 2 -> B/W