Oscarus

Actually, in the anime, during his battle with Leon, he was presented as "Lance of the Elite Four", signifying he had lost the position. 

But now I check Bulbapedia, and he apparently in the last episode was referred as "Champion of Kanto and Johto"... so you're right. 

I'm not watching anime regularly, and spoilers on YT and other sources are so common it's nearly impossible to avoid. And my friends spoiled it to me too... 

So, we have, in fact, all Grand Champions of canon regions... plus one, semi-random Trainer. 

To be fair, I'm not mad. 

Actually, I think that the list is perhaps the most accurate as it can be. Maybe I wanted a Frontier Brain like Anabel, Brandon or Palmer to appear, but... The list makes sense - all current and currently-known Champions are there, so it makes sense. 

One could argue with the rankings within the list, but since they (probably) didn't battle each other (expect some matches in the past, like Alain vs Diantha, Ash vs Leon, Ash vs Iris and Leon vs Lance), the more they battled, the higher they are.