I'd say that the top six goes something like this, and assumes a general lack of character reluctance in going all out with intent to kill:
1. Mewtwo
2. Ness
3. Lucas
4. Ganondorf
5. Pit
6. Kirby
Before anyone asks, reasoning behind the picks:
Mewtwo: Canonically the strongest Pokemon in his series; was outranked by Arceus 'til X/Y. Pretty much the entire cast has no defense against psionics. When you can feasibly mind-wipe your enemy, well, Mewtwo has the upper hand. Mega Forms essentially make Mewtwo stupid powerful. Finally, to end it, an arsenal of extremely powerful techniques allow him versatility to tackle most any genuine problem. Assuming Recover is a thing, which it should be in any serious fight, longevity puts most bad matchups firmly in his favor.
Ness: I actually had a bit of trouble figuring out who was stronger between Mewtwo and Ness, but I gave the #1 spot to Mewtwo since I'm fairly certain Mewtwo is the stronger psychic. Ness hits the #2 spot 'cause not only is he an incredibly strong psychic, but also has experience fighting inorganic enemies, like robots. Similarly, enemies like Game & Watch would prove little trouble since, well, when you've fought the embodiment of evil and won, an LED character won't cause much trouble. Also has the support and power of the Earth. Seriously. Watch what happens when you finish Magicant. Those are some ridiculous stat boosts. Like Mewtwo before him, Ness also has access to recovery and defensive techniques.
Lucas: Similar to Ness, though I feel a weaker psychic compared to #1 and #2. Also with experience fighting inorganic enemies. Contrary to popular belief (and his portrayal in Brawl), Lucas is NOT a crybaby. The whole storyline surrounds Lucas' loss and grief, and how he comes to term with that and grows up. Play the game sometime, it's great. Like Mewtwo and Ness before him, also has access to recovery and defensive techniques.
Ganondorf: This should be a no-brainer. Effectively immortal, has 33.33% of a MacGuffin called the Triforce of Power, is just a total villain through and through. Sadly, demonic villain with MacGuffin or no, I feel like if Mewtwo, Ness or Lucas wanted to, they'd have little issue with him. On the other hand, Ganondorf is still no pushover, since he could dominate anyone else on the list. Before anyone else mentions 'Well, Link could beat him!', the official timeline insists that one of the branching points of the timeline is where Ganondorf actually KILLS Link. The Goddesses have to flood Hyrule to keep Ganondorf from being the badass he really is.
Pit: Lots of versatility in his arsenal and can fight effectively at all ranges, plenty of experience fighting opponents of all kinds of crazy, effectively immortal and also has the immediate support and assistance of various Gods and Goddesses in his own world without need for preparation, typically. The only reason why Pit is here and not a certain someone else from his series that starts with an H (and he'd be #1) is simply because we're discussing current or past fighters in Smash. I just can't really see Pit overcoming the brain-melting psionics from the top three, and no matter the range advantage, I figure Ganondorf has the match up on lock.
Kirby: Takes the powers of enemies and utilizes them for himself. This all in itself makes him extremely effective as a fighter, but he also clearly has skill outside of power combat and betrays an amazing strength and resilience that isn't obvious at first glance. One could argue that Kirby would have a strong chance to topple the other five fighters, and that's why he's even on the list to begin with. Plus, it's debatable if he even has a brain but, even so, Mewtwo, Ness and Lucas shouldn't have an issue pummeling Kirby with genuine psychokinetic attacks and a versatile arsenal.