What is the highest level Pokémon that a npc can have in ANY GAME?
The highest possible level is against Lass Diana in Black and White 2. If you trade her a level 100 Excadrill, you will then have the option to battle her later. Choosing to do so will make her use that level 100 Excadrill, and thus she has the highest levelled Pokemon of any NPC.
Hope I helped!