Volcarona and metagross are ofc great pokes, though it's a pain in the ass to train them and I think Reborn challenges you at using different Pokémon. Magnezone is good, though as of EP 16 you can't obtain It until very far with the storyline. Early game gulpin/swalot Save your Life, and You can easily get a piloswine and then a mamoswine, which are of great help. The mistery egg Pokémon is usually a very helpful choice too, but then I would suggest you just choose some Pokémon you like, train them and see how It goes, I had wonderful results with klinklang, scrafty, meowstic etc.