Level(Maple Story Meso) 40-45: Continuing Monster Carnival is your safest bet. Now you can get into both the Monster Carnival, and the Ludibrium PQ much easier at these levels. They also give a lot of exp. The Ludibrium PQ actually gives more overall exp than the A rank reward at the Monster Carnival even after the exp decrease, but the Ludibrium PQ is somewhat more difficult to do.
Level(Maplestory powerleveling) 45-50: At this point, the Monster Carnival tends to get a little slower. So some alternatives are in order. The Ludibrium Party Quest is still a good alternative.
Level 50-60 in Maplestory: Now that you have made it to Level 50+, you could start the Ludibrium Maze PQ. It is a great way to earn some exp. It's also a great way to get some great prizes. Cellion, Lioner, and Grupin. With your Storm Breath ability, you can kill them with some ease.