Yabuki's art is among the greatest, but I find it sad that he hasn't written any of his own stories since Black Cat. It definitely shows. TLR and Mayoi Neko were many steps down, even if the art was better. Most notably, look at the shift of protagonists. Rito is uninteresting, and the Mayoi Neko hero is so boring that I even forgot his name. Yes, TLR Darkness gets better, and is one of the first harem mangas to come out and actually "be" a harem manga instead of tying the hero to one girl who everyone knew he was gonna get with since the beginning. The outcome is still obvious, but it's a step in the right direction.
Personally, I'd say Yabuki qualifies just with Black Cat. The beginning was a little shaky; he obviously didn't know exactly where he was going with Train as a character to begin with. Beyond that, though, it sets a good tone and sticks to it. It's definitely one of the best pure shonen mangas out there, standing along side One Piece, Dragonball, etc. And it's shorter than either of them. I don't have a problem with long-running manga, but there comes a point when new readers just can't get into it due to length. I quit reading the three big ones (Naruto, Bleach, One Piece) because they were getting boring to me. Yes, HSDK is the same, but at least I can relate to the main character. That makes a bigger difference than you'd think.
/Defense of favorite artist over.
He's not my favorite mangaka (that award goes to Syun Matsuena [HSDK]), but he and Tony Taka are by far my favorite artists.