Hmm, interesting. Let's see here...
Elite Four #1 - Welcome to the party!
Lorelei
Lapras
Dewgong
Jynx
Walrein
Glalie
Lapras
You have no idea how many times I got absolutely shredded by Lorelei in RBY. The first time I played, I started with Charmander and had way too many Fire and Rock types when I reached the E4, so Lorelei destroyed me numerous times with nothing but Water moves. The funny thing is, I think I've only lost to Bruno once, and that was only because I had one Pokemon left with something like 4 HP and no healing items remaining. (Thank you again, Ice Queen!) As such, she has earned my respect enough to be the official greeter of the ultimate E4.
Elite Four #2 - Oh, so you made it to me? Let's dance, kid.
Karen
Crawdaunt
Houndoom
Umbreon
Shiftry
Mightyena
Absol
Karen never was all that tough of an Elite Four leader, but she was still pretty cool in a dark (ha ha) sort of way. She used nothing but Dark types, yet emphasized using your favorites rather than just picking strong Pokemon. Makes you wonder about her personality. She was no leader, but she still wins a spot on the team.
Elite Four #3 - Sorry you had to come all this way just to lose.
Steven
Skarmory
Magneton
Steelix
Forretress
Aggron
Metagross
Steven was a neat guy, in my opinion. In RS, he watched you grow from a beginning trainer to a powerful one who could challenge even the Champion. But he still didn't let up, throwing a strong team of Steel-types at you to test your mettle. Even though he wasn't all that strong of an opponent when I battled him, he was still strong and interesting enough to make this E4.
Elite Four #4 - It appears you have some skills. But let's see how much you can focus, child.
Agatha
Misdreavus
Shedinja
Dusclops
Banette
Wobbuffet
Gengar
Ah, yes. Agatha, the original Ghost type master. How could she not have a slot on this team? With all the knowledge she's gained over her many years, she's certainly a worthy adversary for any trainer who wishes to challenge the Champion...
Champion - I've been expecting you. Do you think you have what it takes to triumph over this league?
...Then show me!
Lance
Dragonite
Kingdra
Altaria
Flygon
Salamence
Dragonite
What Elite Four Challenge would be complete without Lance? The top E4 member of the Indigo League... the Champion of Johto... and the best Dragon trainer in the land. Hoenn teams aside, there's no way this guy should ever be left out of the loop.