It sort of depends on the manga genre. To-love Ru was really good all the way through, and Darkness is continuing the trend, but there is no real plot whatsoever (maybe with Darkness?) and the series basically lived on fanservice and character trait jokes. Hayate no Gotoku is great because you get that 4-koma feel in a fleshed-out story. You don't have any real expectations of it, yet it delivers random, funny jokes with great aplomb. Bakuman continues to impress.
If you're talking about pure shounen, then there aren't many examples. Eyeshield 21 got better and better in basically every aspect, no doubt there. Hitman Reborn went from being pure, mediocre humour to a battle manga with great art. I actually look forward to it these days. Kenshin would've been good if Enishi was actually someone from the Shishio arc. Negima is getting good again. Kenichi is sort of floundering, but it just finished a very good arc, so that's to be expected.
Short manga tend to be great. I loved Miman Renai and Welcome to the NHK, Busou Renkin impressed me, and Bamboo Blade (it's not particularly long, like 120 chapters, of which I've read like 110, 5 or so untranslated) was very good.
Code Geass's manga versions were horrible and I could barely even read them. D.Gray-man got awful after about 5 story arcs, and I could barely follow it, though I can see why it is still publishing. Prince of Tennis just pissed me off to no end, and I ended up reading it just to laugh at the absurdity.
There seem to be more good long mangas than bad ones. I can see why people like Bleach, but in my very subjective opinion, it's really fallen off the track. It then continued further off the track and veered off a sheer cliff, at which point it fell onto a submarine, sustaining plenty of injuries. This submarine then submerged, but was attacked by power creep torpedoes, and sunk in despair into a deep, deep trench. They might be able to rescue it if they give Ichigo enough power to evaporate all the water on Earth instantaneously, which at this point would not surprise me in the slightest. Just kidding, the amount of power added to this series is so great it's actually boring.