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To me, difficulty isn't a factor but their impact/presence is. As far as I knew, in Gen 1 the Elite 4 were treated as these secretive trainers that were almost never seen, so them being absent in that context made sense. In Gen 2 Lance was then active in the story going to stop Team Rocket. Invading the base with him was pretty cool. I don't know about if this was a difference between Gen 3 to ORAS, but in ORAS Steven had presence in the story and helped you too. Can't speak for Emerald and Wallace. I don't remember much about Cynthia which to me means she probably had almost no presence, but its a woman so she's probably gets propped up way more than she actually deserves. Alder is an absolute joke who gets BTFO by N, can't speak on Iris since I never played B/W2. Diantha also has no presence and is barely memorable. If we're counting Kukui, then I think he and Leon are pretty good and Leon gets extra props for being a big part of the Battle Tower. Trace is just a Blue analog so it drags him down.
tl;dr
>High Tier
>Lance
>Steven
>Leon
>Kukui
>Mid Tier
>Blue
>Cynthia
>Low Tier
>Trace
>Diantha
>Joke Tier
Alder
>Unsure
>Iris
>Wallace