Best Mario & Luigi: Bowser's Inside Story Bowser Locations

What are the best areas inside of Bowser in Mario & Luigi: Bowser's Inside Story? Let's find out. I'll only count the major areas with actual levels inside of them. So with that being said here is the list.
The Top Ten
1 Flab Zone

The Flab Zone..., visually, it might be the least attractive of the areas, at least on the DS original, but it's still in my opinion the most fun and interesting of the areas visited. Being the third one visited while Bowser is in Bowser's Castle and his brainwashed minions forced him to eat a bunch of cake, causing him to get fat, Mario & Luigi must then travel to the flab zone, you know the area where Bowser has all his fat, and we must find a way to make him burn all those calories. It's an all in all somewhat a creepy experience, but in a really good way! As we're playing this inside of Bowser's Castle, the music is an "inside Bowser" remix of that song, which adds up to the experience, as it sounds kind of distorted. It just makes it all the best of the five major areas visited inside Bowser.

2 Airway

Now this are sure has some atmosphere and climax to it, as it's the last area visited inside of Bowser, nearly at the end of the game, when Bowser has bested the Icelord Midbus, but the dark star has escaped to inside of Bowser and is stealing his DNA to become all the more powerful. So it's Mario & Luigi's job to chase it down. Bowser can also breath in the icy breath coming from Midbus' corpse so that the inside of Bowser freezes and Mario & Luigi can traverse on ice (similarly to the Luiginary Sun in Dream Team). Plus the music...! A remix on the Deep Castle music in Peach's Castle, it's an even better version of the already classic song! So overall another amazing area!

3 Pump Works

The second major Bowser area of the game, set on Plack Beach, when Bowser has just defeated the Blooper statue boss battle. He can then drink the water shooting out of it, which can fill Bowser's body with water, which can help the Mario Bros. to traverse areas inside of the Pump Works. I don't particularly remember that much from this area, as it's one of the first, but it was pretty fun with the gimmick. So it's a great area.

4 Trash Pit

The absolute first area of the game. Not just inside of Bowser, but the first of them all. Right after Mario & Luigi have been sucked into Bowser's Body, they end up in the trash pit, and must find their way out. It's not a huge location, but big enough to make it onto the list. We find hammers, other toads that got sucked up, etc. Not much to say about it. The music is a remix on the Underground Tunnel, which makes it feel a bit empty and bland, but it's not a big problem here.

5 Energy Hold

Energy Hold..., it's pretty frustrating. After the amazing level that was the Flab Zone, our next area doesn't even have half of the fun. It also contains the same remix of the Underground Tunnel as Trash Pit, but that makes it even more bland and boring. It's essentially just a level where you get all the keys to traverse to the main area. Plus the gimmick is super frustrating. Bowser can move in front of a light so that certain blocks appear and disappear inside of the level, but it's a chore to change between them. So it's easily the worst area of the game.