Naruto - I think the earlier stuff is quite poor, where the series was still working out what it wanted to be. The animation from this period looks pretty dated now, too - and I agree Naruto's personality here is really abrasive, and it put me off the show for a long time. I finally got into it by starting with Shippuden though, which I think stands in a league of its own. Yes, there's still filler - and it's probably some of the worst filler in anime to date - but there are particularly arcs that really stand out. The Akatsuki are brilliant villains and the Naruto vs. Pain sequence in particular is a real gem.
Bleach - I've never quite understood the love for the original Soul Society arc here, as I think what comes afterward with the Arrancar arc moves through a lot of the same story beats with increased bang for you buck. What you're here for is endless battles between immensely powerful individuals, and I'd argue that Bleach often does this better than any other Shonen series. The show has many sins - not least padding out every episode with a three minute re-cap every single time, but the central premise of fantastic match-ups in the various battles always kept me coming back.
One Piece - the one I've never really been able to get into. I followed it for about a year in Weekly Shonen Jump (probably a bad jumping on point) and found it impossible to get into - the cast just seemed too large, the style - both artistic and story-wise - too wacky and unwieldy - so it always baffled me how it continues to sell so well. More recently, I finally watched the first 25 episodes and enjoyed that - so I'm starting to see why it works, but I think I'll need to sample a bit more before I really know how I feel about it.
Magi - I think, is probably one of the best of the current crop, and it seems to sell pretty well too. I really hope Kaze get the second series out here too, as I really enjoyed the first arc. It just really shines with high production values, strong characters and a distinct aesthetic.
One thing I'll always say about Shonen series is, what works for me is where the arc/process is very clearly outlined - show me a bunch of ridiculously powered up characters, tell me how many there are of them, what rank/power level are they? And away you go. Shonen thrives on its capacity to deliver those kinds of dream 'who'd win?' match ups - and if it can keep scratching that itch, it will always have a certain entertainment value.