Only looking at in game canon and elements of gameplay that prove consistent without contradicting it (So no Hyrule Warriors Link with the Ocarina, Wind Waker, Fierce Diet Mask and so on) and tend to be within the realm of realistic situations for these characters (No 99 live Marios) here are my thoughts:
S Tier:
(Spoilers but let's just say the Monado has quite a lot going on and the ending gives Shulk a huge edge)
(Kirby stands far above other God Slayer characters due to having massive adaptability, massive potential, and has displayed within his own games some degree of planet busting capacity, something which isn't really seen in general by any characters within Smash Bros.)
(Because the character is Rosalina AND Luma, you get a pretty solid amount of juice out of these guys. The Luma are capable of doing insane stuff like forming planets and stopping bllack holes when they work in numbers, and Rosalina's powers while hard to measure within canon at the very least are on a cosmic scale)
(Similar levels of destructive power as Kirby but with much less potential on the cosmic scale. You don't lose to Kirby repeatedly and expect to be above him. The one thing he has going over Kirby is that he's fully grown and knows a ton better than Kirby does.)
A Tier
(Young Link is a time traveler with a mask that turns him into a god, the best arsenal of any Link, experience from saving the world twice, pretty solid magic, swordsmanship, and so on. That all said, Young Link marks a gap where you stop seeing planet busters)
(Working with Super Sonic having 50 Rings since its the most consistent number shown and Rings in general are pretty BS when it comes to how they work within canon and sometimes even gameplay. Sonic fights anything ranging from giant mechs, to ancient monsters. to even Magic Users. Super Sonic gives him that extra push to having the potential to take out cosmic scale foes around which is a pretty big deal, though it's limitations prevent him from being a full on godslayer.)
(Fights Cosmic Horrors with powerful PSI, capable of creating various barriers, teleportation, blasts of Psychic energy, can absorb energy, and heal himself. Can endure a battle with a galaxy destroying horrror monster just fine. Carries around his trust Bat and his projectile deflecting Franklin Badge. )
(Pit is a Pretty standard God Slayer with a huge line of weaponry behind him and all sorts of magical items too. Dark Pit is pretty much the same but with the whole "I DROPKICKED A BOSS" as well. Honestly not sure whether he's just below Ness or a good amount above.)
B Tier
(Shock! Ganondorf this Low!? Yeah as tempted as I am to put him higher, Ganondorf himself tends to get wrecked repeatedly by kids with swords and light magic in general. In Ganon for he's a much bigger threat capable of massive conquest but he has a tendency to get killed and/or sealed a ton.)
(Anyone ranking Palutena all the way on top needs to stop looking at the title of "Goddess" for a second and look back at her boss fight in Kid Icarus Uprising where she tended to get killed pretty fast. Greek Gods are powerful but only slightly less prone to getting killed by something that'd kill most humans.)
(Sprinting pile of missiles, bombs, and lasers covered in Power Armor with lots of health tanks and massive overall mobility. Even without the suit she's still Superhuman in terms of her physical ability. She fights all sorts of deadly space creatures and then gets out there before the planet explodes time after time.)
(Jumping pile of guns, jetpacks, bombs, time freezing powers, lasers, flamethrowers and anything you can think of combined with his E-Tanks for durability. That said he's got a screaming weakness to lava and spike pits.)
(Seriously, Ike is one of the best Units in the series and strongest Lord in general. No other Fire Emblem character in Smash can really solo entire games the same way Ike could)
I can already smell the controversy as far as Sonic, Ganon, and Palutena go.