Big stadium and small court in nimbasa city are good places to train because you can go there every day and rebattle them. Their Pokemon's levels are in the 60's so they will give you a relatively large amount of exp.
Another good place to train is the royal unova in castelia city. It can be ridden daily during the evening and the Pokemon there are in the 60's.
You can also rebattle your rival daily in the ex team plasma base in driftveil city and his Pokemon are level 65-67.
You can also do a daily double battle at striaton resturant against the ex-gym leaders and their Pokemon are level 65-67.
You can also do black skyscrapper/white tree hollow in black city/white forest and you can rebattle this as many times as you want. I would recommend this as the top spot to train as the Pokemon vary between 50 and 80 as you progress so you can make it easy for yourself or you can make it hard and get extra exp.
For all of these, I would recommend that you give your Pokemon the lucky egg which can be found in celestial tower and this will multiply your exp by 1.5x.