I think the fanbase of every medium in existence gives themselves a bad rep.
My thoughts on those I know:
Sorachi is hilarious, though #1 is maybe a bit of a stretch. I wouldn't bother to dispute it. The fact that Gintama hasn't grown stale (as far as I know anyway) after all this time is a testament to...something.
Oda is creative and the king of worldbuilding and foreshadowing, but he's gotten self-indulgent. Still up there. I think some of my issues with One Piece also stem from the fact that this guy's probably restricted by Shonen Jump a little bit too. He does a good enough job making things work out. Or did.
Toriyama's memory. 'Nuff said.
Arakawa. No real complaints I guess. Though to be fair I've yet to get into much of her work beyond FMA. Similar problem with her in regard to shonen. FMA wasn't published in Shonen Jump, but still.
Araki. Well. JoJo's no literary masterpiece, but it's not trying to be. So I guess he knows what he's doing. All dat gar though. So manly. Their muscles look like they're gonna rip through their skin any minute. I wanna be like Old Joseph when I grow old.
Inuyasha a shit.
I'm not far into the Tokyo Ghoul manga, but I dunno, #7 seems a bit much for this guy as he's basically a rookie (kind of...). I don't doubt that the shitty anime adaptation didn't do Tokyo Ghoul justice, but yeah I'm a bit doubtful about this placement.
Kishimoto stopped giving a shit so no. But unsurprising that he ended up in the top 10. Something would be wrong if he didn't.
Conan is still alive and well so I guess its mangaka doing something right.
Togashi is good but he's too lazy. I've heard that he has health problems too, though, but I've heard a lot of other things too. Unlike Oda his works seem more thrown together too, though I suppose he should be lauded for not ending up with a million and a half plot holes in his works despite his laziness.
Isayama needs to draw better. Shingeki no Kyojin's artwork has improved but it's still amateurish looking, last I checked anyway. Doesn't help that SnK kinda sucks cock imo. But it's also his first work so I'm a little easy on him. I trust that he'll get better with time. If not, oh well.
Kubo...yeah. Kubo.
How come they didn't include Vagabond among Inoue's work? These guys seem to follow seinen as Tokyo Ghoul's on there. <del>Or is Vagabond not edgy enough?</del> Slam Dunk's not too impressive but I see the appeal and am a victim of said appeal. Inoue's also a very good artist.
I think Obata's a good artist. Death Note had pretty good art. Bakuman seems alright. Many of the downfalls of Death Note weren't really his fault (or Ohba's, rather, as I believe Ohba was the one primarily responsible for the story). Not that great though.
The Fairy Tail guy once admitted that he did little to no planning when writing Rave Master. I've not read Rave Master, but I've read Fairy Tail, so I can see that. He gets ripped on for copying Oda, but in some ways he seems to be the antithesis of Oda.
I've heard good things about the Touch guy and the Honey and Clover guy. I'm going to assume they're true and say good on these guys for including these mangaka.
The Nisekoi guy is a fucking troll. He deserves an award for that alone. Kubo's got nothing on this guy. The only reason I've read as far as I have into Nisekoi is because I love finding the corresponding reddit threads where everyone's complaining about the meandering and the bait. Holy shit, the bait. It's fucking wild. Hilarious. That said, I too say fuck this guy, but at least he drew kosaki. But anyway he'll be around for a while. Maybe.