I'm a somewhat avid manga reader. Since we're talking about the most popular ones... let's see:
Naruto is right now quite bad. It started as a really promising manga; Wave Country Arc was great, the Chuunin Shiken Arc was interesting — although common place, it was interesting nonetheless —, the rest of the Part I was decent overall. Then Part II started, and it became too emotional, the old themes were abandoned in favor of directly contradictory ones (from "hardwork and confidence that surpasses genius" to the "Child of the Prophecy" garbage), most villains are just misunderstood people with sob stories, etc. Pain Arc was horrible, Nagato being defeated by a book was horrendous, and the arc made Jiraiya's death somewhat meaningless. The current arc is the worst by far, Edo Tensei being used as a plot device, Kabuto being an horrible villain, most fights being obvious and predictable, the list goes on.
Bleach was promising once as well. Soul Society Arc was definitely good, but after it the pacing it went downhill. Hueco Mundo Arc late parts (Fake Karakura, Aizen's invasion) was nothing short of atrocious. The pacing — though the author may think of it as cinematographic or something — is also hard to bear, with completely needless shots of the characters' faces in almost every chapter; at least now Kubo isn't doing it as frequently as before. Still, I like Bleach better than Naruto, probably because the latter was a greater disappointment.
One Piece is the only one that isn't garbage, in my opinion. It isn't the best manga ever, obviosly, it isn't on the same level of Berserk, Monster, Vagabond, Planetes, that should go without saying. It isn't even the best shonen ever, for example, it isn't on the same level as Full Metal Alchemist or Shingeki no Kyojin, in my opinion. However it is still good. It was even better a few years ago, in the Shabondi Archipelago.
In the Shonen Jump magazine, one also has to mention Toriko. It's a great manga, with lots of absurd scenes and all around parodies of the shonen action genre. An unexpectedly enjoyable and promising manga. Also, Hunter x Hunter; a great manga, despite Togashi's scribbles and general laziness. If Togashi wasn't a hack, it would be 150% better.
Lastly, since it seems to be really popular now, there is Fairy Tail. It's an enjoyable manga, if you don't read it seriously. If you read it seriously... let's say, analyzing it from a literary standpoint, it's real garbage, way worse than Naruto and Bleach. Otherwise, you can read it and enjoy the abundant fanservice, if you're into that, and the predictable plot full of "Nakama Punches" and whatnot.
Edited by IntusLegere on 9/27/2012 7:21 PM PDT