From least favorite to favorite, here they are, and why I think they're good (or not):
Yugioh= Lame & Boring. Tied for my least favorite Jump manga with DBZ (It was good when it dealt with ancient Chinese, Egyptian, & japanese stuff, but u kno sum1's running out of ideas when they use a yo-yo as a weapon & a plot device)
DBZ= Worse than Yugioh (It's old. VERY old. It ended a long time ago; why can't it just be left to die? It's not even that good)
YuYu hakusho= Fairly good so far. The animation style isn't my favorite, but that doesn't take away from the sheer enjoyment of the manga
One Piece= Excellent. Humorous, well-driven storyline, & good characters. And a very unique art style
Naruto= VERY good. It can be hard to follow the action at times, but that's being nitpicky. I have no trouble seeing why Naruto is so popular, both here & in japan (I myself download the new episode each week). Probably has my favorite artwork of all the Jump manga
Shaman King= Definately one of my favorites. The storyline is unique, the characters are very well done, each with a very different & original personality, simple yet effective art style. And very different from the Shaman King anime (which is also very good, if you see it in japanese. the dub is horrible), so it gives me something I haven't already seen (unlike Naruto, who's anime is almost identical to the manga. This is the only reason Naruto & Shaman King aren't tied as my second favorite)
And my favorite Jump manga is...
Hikaru no Go=It's the storyline. The concept of a manga based on an ancient Chinese board game sounds so boring, yet I find myself flipping to HikaGo first thing whenever I get the newest issue. Best part is, it doesn't have fancy monsters or powerful technology or uber-mystical stuff (it has Sai, but he just tutors Hikaru) like Jump's other game-based manga (Yugioh). And the games of Go played aren't for "world domination" or "saving the world"; nothing but the player's pride is at stake. It manages to retain reader interest & have compelling storylines & characters without all the add-ons that are present in the other manga...which is what makes it, by far, the best manga in the North American Shonen Jump.