there are a lot great places to train your pokemons. i narrow it down to the top 5 places i would train.
No. 5- Twist Mountain. this mountain is a great training place.but this place only scores No.5 to me because of the lack pokemon. the 2 pokemon that will pop up most often is Boldore, and Gurdurr. those both gives a lot of exp but boldore's sturdy ability gets in the way of 1 shot KO. gurdurr, on the other hand is fairly easy to kill and gives a lot of exp. also, if you train in the winter, a cubchoo or a crygonal can come out. cubchoo is useless on the other hand, crygonal gives plenty of exp. Twist mountain doesn't appeal too me too much, but it is a great place to train grass, water, and fighting types....but mostly fighting.
No.4- Relic Castle. if you go deep in Relic Castle you may fine lack of pokemon, but at good lvl. if you are in deep enough, mabye 2 floors or so, you may run across some Krokorok at lvl 40 or so. so, as long as you keep finding some this palce will be great for grass and water type. it stays at no.4 because it doesn't have a healing area. so if you were to train here, bring lots of potions and revives
No.3-Route 10. this place is mostly jammed with a variety of pokemon. but what makes this place at no. 3 is Buffolant, amoonguss and throh. those 3 pokemon can be found on that route, and as long as you kill them, they give tons of exp. for example, my gothitelle lvl 41, went up to lvl 47 in a matter of minutes. but this place sticks at No. 3 because they mostly show up rufflet/vullaby, and foongus. also, buffolant has a high attack and a very good move, killing most of your pokemon easily unless 8 lvls higher or so. this place is probably a nice place to train psychic pokemon and somewhat of fighting pokemon
No.2- Route 7. this route and no. 1 route was a hard decision. but i decide this is no. 2. route 7 is one of the best place to train lvl 35-45. With zebstrika and Watchog giving tons of exp, this place is a good training method. not only that, but there is a healing center close by. if you train here, its best you go in the double grass, as the double grass has higher lvl pokemon then the single grass. this stuck at no.2 because of a very important reason. it sprang too much foongus.dearling and in the winter, cubchoo! those pokemon give you barely any exp, and they are so common! i like this place, but i rather go to no.1. this place is a good place to train any type of pokemon except water (a lot of thunder and grass type)
No.1- Route 9. This route was probably the best for me. it had 2nd evo of other pokemon, giving much more exp. pokemon located there are duosion/gothrita,pawnliard,garbador,liepard, etc. also, duosion/gothorita are the most common pokemon to arise here, so you earn more exp.this place is also near a town, so you can heal yuor pokemon pretty quickly. the only things that would bring you down would be a minccino, other than that this place is a very good place to train. the only types you dont wanna train here is psychic. other than that, every class is good here especially Bug types
that is my top 5 until next time.