Well, My favourite place to train was at the bottom of Island 7 because of the house you could heal in and the 2 trainers;
Cool Trainer Leroy
Rhydon Lv 47: 2,053 EXP
Slowbro Lv 48: 1,686 EXP
Kangaskhan Lv 47: 1,762 EXP
Machoke Lv 48: 1,501 EXP
Ursaring Lv 50: 2,025 EXP
Total: 9,027 EXP
Rewards: 1,800 En
Rematch
Rhydon Lv 52: 2,272 EXP
Slowbro Lv 54: 1,897 EXP
Kangaskhan Lv 52: 1,950 EXP
Machamp Lv 52: 2,149 EXP
Ursaring Lv 55: 2,227 EXP
Total: 10,495 EXP
Rewards: 1,980 En
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Cool Trainer Michelle
Persian Lv 47: 1,489 EXP
Dewgong Lv 47: 1,771 EXP
Ninetails Lv 48: 1,830 EXP
Rapidash Lv 48: 1,974 EXP
Girafarig Lv 50: 1,596 EXP
Total: 8,860 EXP
Rewards: 1,800 En
Rematch
Persian Lv 53: 1,680 EXP
Dewgong Lv 53: 1,998 EXP
Ninetails Lv 54: 2,059 EXP
Rapidash Lv 54: 2,221 EXP
Girafarig Lv 56: 1,788 EXP
Total: 9,746 EXP
Rewards: 2,016 En
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Total after rematching both: 20,241 EXP
and even more if your Pokemons got the EXP.Share, Lucky Egg or been Traded.
Hope I helped