Then how did Kirby get inside if it's completely closed in?
Through one of those magic star doors that work more like portals than actual doors.
Anyway I see that people have been posting their own lists, I haven't because I can't really settle on an order in the top 3 but here it is in my honest opinion. Just keep in mind that the top three are in no particular order.
S Tier
-Pretty quick, can transform into Super Sonic to drastically increase his durability. Not to mention teleportation and wisp powers for variety
-8 inch tall, 15 pound powerhouse! Numerous abilities that synergize well with the Warp Star to deliver light speed destruction! Has several invincibility options
-Unreal durability, great mobility, and a varied and powerful arsenal. Not to mention her incredible strength or her overpowered gunship
A Tier
-Completely invulnerable to non-holy weaponry, not to mention he is surprisingly agile, has dark magic for days, is skilled with a sword (or two) and can warp. He can also become invincible in darkness, so can we get a read on how well lit the arena is?
-Very fragile but can create lots of black holes very quickly, getting them in to position is another story though. She has decent defense in form of her forcefield.
-Composite Link has 3 days of invincibility. 3 DAYS! Not to mention excellent marksmanship and swordplay. He also dabbles in magic.
-Ether manipulation. That is all. The reason he doesn't get any higher is that he doesn't really have any reliable defenses and he's not that quick.
- They have almost the exact same abilities (Dark Pit doesn't get either of the sacred treasures) and those abilities include a huge arsenal of specialized weapons to take down foes.
-Fast enough to fly around the galaxy, and powerful for the puffball he is. His sword can generate powerful sword beams, he has a form of teleportation, and he has mild control over elemental forces.
-The so called goddess of light has a wide range of powers (including the powerful Mega Laser) as well as some attacks of her own. They're pretty limited though.
-Has OK telekinetic abilities and a wide range of assorted attacks including a teleport he can use 32 times and the powerful Psystrike.
B Tier
- A giant arsenal of assorted weapons...that's about it.
-Master of practically all forms of combat, most notably his powerful magic such as Aversa's Night or Thoron
-Pretty limited set of moves but is powerful where it counts, PK Rockin' Ω in particular is incredibly powerful
-OK all around, relies heavily on equipment and power-ups to do damage
-An above average swordsman with invulnerability to all direct non-manakete (dragon) attacks
Bam that covers everyone from S-tier to B tier