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Protip: Alder is actually challenging if you don't grind for him and ruin it.
Fact: Gen2 Lance despite being Lv50 and having three Dragonites was more challenging than gen1 Blue, because the latter's movesets were absolute shit. FRLG fixed Blue.
I'm glad someone else agrees that Iris was the best team, with half (not full) Dragon, decent species and good movesets, plus good items in Challenge Mode.
Something that bugs me here is that Alder for BW is included over Ghetsis, when Ghetsis is the initial final boss. If Alder is included (if it's about the Champion position), Red should be too alongside Lance, or if Red isn't included, Ghetsis should be instead of Alder (if it's about the final boss role).
For me the difficulty from highest to lowest is:
Ghetsis
Cynthia
Iris
Alder (I never grind for him, so a slight semblance of difficulty is added by the fact that his team is level 75-77 vs my 55-65s)
Steven
Lance
Red (Same case as Alder, my team is usually around level 60 to fight his 80s)
Wallace
Hau
Kukui
Blue
Diantha
Trace (haven't played legpiss but OP starter + Exp. Share + 8 levels lower than Blue = joke)