If you mean in Pokémon Black & White, there are a few main places to train your Pokémon:
• Nimbasa City Domes - All of the trainers there will have Pokémon between levels 61 and 68.
• The Royal Unova - This is a cruise ship in Castelia City that's accessible in the evenings. The trainers here have levels between 30 and 35. If you can find and defeat them all before the ship returns to the dock, you'll get a prize depending on how many battlers were on board at the time.
• Game Freak - A man named Morinaga there has Pokémon between levels 74 and 77.
• Undella Town - In the spring and summer, Cynthia has Pokémon between levels 75 and 78.
• Audino - They're everywhere that shaking grass is present. The highest-leveled Audinos are around 60 and can be found in the Giant Hole, where you'd find Kyurem.
And, of course, if you're looking to continue the story, in HeartGold and SoulSilver, there's the whole Kanto bit; and in Black and White, there's northeastern Unova, where you can finish the northern Unova loop with Village Bridge, Lacunosa Town, Undella Town, Black City or White Forest, and Marvelous Bridge, and every feature in between.