I know that there's already an answer, but I'd like to include my own.
Viola - Lvl. 11 through Lvl. 13
Grant - Lvl. 24 through Lvl. 28
Korinna - Lvl. 27 through Lvl. 34
Ramos - Lvl. 35 through Lvl. 38
Clemont - Lvl. 39 through Lvl. 41
Valerie - Lvl. 42 through Lvl. 44
Olympia - Lvl. 47 through Lvl. 52
Wulfric - Lvl. 56 through Lvl. 60
I like my Pokemons' levels to be a bit over the gym leaders' Pokemons' levels, but not too much because I want to be able to beat the gym leaders with one try but still have a little bit of a challenge because they're gym leaders.
I know you didn't exactly say that you wanted recommended levels for the Elite Four and the Champion, but I figured I might as well.
- Malva/Wikstrom/Siebold/Drasna - Lvl. 65 through Lvl. 67
All of their Pokemon are within Lvl. 63 - Lvl. 65, so having your Pokemon be about 1-2 levels higher than theirs is reasonable.
- Diantha - Lvl. 66 through Lvl. 70
All of Diantha's Pokemon are within Lvl. 64 and Lvl. 68, so again these levels are reasonable.
With the new EXP. Share, you don't have to grind too much (if at all) so it's easy to reach these levels, and I find that these levels are fairly reasonable.
Hope I helped!