Favorite mangaka...hm...not sure if I can give an easy answer.
Bleach and Naruto were both pivotal manga for me, as they were the first ones I really read weekly, but I can't really talk positively about the authors behind them at this point. Both had great starts, but I grew less and less interested in them as they went on. The final arcs in both manga were largely forgettable for me. I actually dropped Naruto after one of my favorite characters was killed for basically no reason but a plot device, and I grew sick of how many laws of the universe were just discarded to advance the story to its climax. As a reader, I remember feeling rather insulted with it, and decided not to stress myself out by reading it any longer.
A while back, I'd say Hiro Mashima. The artstyle of his manga (Rave Master, Fairy Tail, and several others) has always been really appealing to me. It's clean, detailed, and I can actually follow the battles without problems, which is honestly more rare than it should be in the genre. He also does a great job with character design, and the comedy is usually pretty great.
Unfortunately...Hiro's story-telling skills are extremely hard to defend. He can draw emotional moments for sure...the problem is, it feels like he forces the story to reach those moments, rather than approach them naturally. I don't really get the impression he has the story planned out very far in advance...which some mangaka can handle well, but here, not so much. He also hates training arcs and extended fights...which is fine, since I also hate them, but it means he usually goes in the opposite direction, with constant nakama power-ups and sudden hero victories after dozens of chapters worth of build-up. I've said it before...but I'd love to see Hiro try his hand at a romantic-comedy manga, since I think most of his faults would be insignificant in a story like that.
I've read a lot of manga since then, and most of them will either excel in one area and fail in another, or start off great all around, but grow boring over time. For example, Yoshihiro Togashi (Yuyu Hakusho, Hunter x Hunter) tells a fantastic story, but his artwork looks terrible to me. Shinobu Ohtaka (Magi) is an amazing artist...but both the characters and plot really bored me over time, especially the current arc.
In the end, I would probably say Eichiro Oda (One Piece) is my favorite current mangaka. He tells an amazing story that still blows me away, even today. He sets plotlines in motion or introduces characters 400+ chapters before they amount to anything, juggles dozens of stories at once, and still manages to create an epic adventure with almost no plotholes. The characters and comedy are hilarious, and I find myself grinning a lot while reading it. The only downside is the artstyle....though I struggle to call it a downside, as it's really only how the women are drawn that I don't really care for. It's probably a minority opinion...but the exaggerated hourglass body shapes really aren't attractive to me, so even canonically "cute" girls look really weird to me. Very minor complaint though.
TLDR: Oda da man!
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