Uh higher level = higher exp, don't really need a list. Highest i remember are the level 75 guys in maps near sand village, and they are very easy, can kill them at level 40-50~ maybe, i remember trying for fun.

But really you don't need them, if you swap your ninjas our during sweeps you can get sooo much free exp, later levels give like 80-120k exp per sweep. All my ninjas are same level as main and i still have thousands of food items left.