Luigi doesn't have the best parts of Mario. This comparison comes up a lot because they're the Ryu/Ken of the series, but Mario and Luigi play quite differently. Luigi wishes he had Mario's mobility and solid traction while Mario wishes he had Luigi's damage per hit and kill set-ups.
Luigi and Doc are a much closer comparison, but I feel this won't be the case in the future. Some players play a very defensive Doc while others play extremely aggro with him. Suffice to say Doc is still being explored on how he should be played, but in terms of statistics Luigi does seem to be a straight upgrade from Doc, even at the end of it all they still end up playing quite differently.
That said, I'm surprised Luigi isn't ranked higher. There isn't a lot of characters better than him and he's top contender of having the best reward off of hit confirms in the game since D-throw is an extremely reliable set-up for virtually anything. He's certainly better than Ness, Mario, and Peach.
I don't think that Luigi is clearly better than Ness, Mario, and Peach. Maybe Mario.
That said, I just swept a local tournament this weekend with Luigi, and I have a few observations on the character now that I've played him for a while.
1) Luigi doesn't get destroyed by zoning. I'd actually put the Villager matchup at 50:50 or even in Luigi's favor. It's too one-dimensional to look at his airspeed and say that zoning is an autowin, because just as Luigi has trouble getting in on zoners, zoners have trouble getting
out once Luigi gets in. Seriously. Unless you can grab Luigi, his aerials are too fast, his f-air is too big, and his jab and grab are too good (just look at his grab speed + range) for most characters to get him away quickly. And zoners almost universally have bad grabs. Pacman and Villager and Link/Toon Link have one heck of a time getting Luigi away once he's in.
Duck Hunt and Rosalina have good grabs, but Luigi deals with the can and Luma quite well, so those matchups are probably 50:50 too just because their zoning isn't as effective against him.
Of the zoners, Megaman is the only one with both a good grab and tools that can keep Luigi out consistently, because of big, disjointed aerials. Fireballs are canceled by pellets, too. Still, that's one out of many matchups.
Walling is still a problem for Luigi, but Brawl Meta Knight and Brawl Marth don't exist any more; you won't be getting walled out in quite the same way you would have in Brawl (overpowered by huge and possibly transcendent disjoints, that is). Instead you have guys like Mario, Diddy, and Pikachu to an extent. Luigi has a much easier time getting in on these guys due to his disproportionately huge, 9% damage f-air.
2) We are in a grab-based metagame right now, and probably will be for a long time to come, maybe even forever without patches, and Luigi is one of the best characters for such a metagame because the best way to deal with your opponent's grab is to grab him first. And if it's a question of grabbing vs. being grabbed, Luigi wins, because even if it's 50:50 Luigi getting a grab yields more reward than anyone else getting a grab.
3) Master perfect pivoting, all you budding Luigis. His perfect pivot sends him really far, and lets him get in on guys like Diddy spacing f-airs with no problem. It's actually kind of hilarious. Luigi is a fairly technical character (jumpless cyclone, learning DI patterns), all things considered, so if you already main Luigi, you should invest the time into consistently landing perfect pivot tilts and jabs. Luigi's perfect pivot to jab may just be the most beautiful thing in the game.