Nobuhiro Watsuki is a mangaka who is far and away most famous for being the brain behind Rurouni Kenshin, one of the most successful Manga and Anime series of the 90s. He's worked on several other titles, however, including Sengoku no Mikazuki (his first published work), Busou Renkin, Gun Blaze West, and he is currently working on Embalming and on diverse continuations of RK, like the prequel to Rule Flame or the sequel Hokkaido Arc.
Watsuki is notable for several reasons, the first of which is his seeming mastery of Multiple Demographic Appeal (which was most obvious and successful in Kenshin, but applies to all his works). Despite being a Shonen mangaka (and in fact one of Shonen Jump's steadiest contributors over the years), he deliberately attempts to place elements in his stories - or write entire stories flat out - that will appeal to other age and gender groups. While this doesn't always work, with Kenshin it created one of the anime juggernauts of the 90s, as the crossover appeal of the characters and story is what propelled it to national and international fame. For his part, Watsuki attributes this to having read a decent amount of Shojo as a lad, and realizing that "boys" and "girls" stories weren't necessarily all that different. This lesson seems to have rubbed off well on his students, too (see directly below).
The second thing he's famous for is something he isn't... directly involved in at all. Watsuki served as the teacher for a certain aspiring mangaka named Eiichiro Oda, and helped that young man solidify his art style and work ethic. Oda went on to create a little manga that you may, potentially, have heard of. If Oda is the God of Shonen, then Watsuki may have to be called the Father-God of Shonen, as it's quite possible that Oda wouldn't have been able to get into the position he did to publish One Piece - or pick up the skills necessary to make it as good as it is - without first working for and learning from Watsuki.
Watsuki had a similar effect on Hiroyuki Takei of Shaman King fame; Watsuki, Oda and Takei are said to maintain a close friendship to this very day.
The final thing he's being famous for is being a bit of a history nut (despite saying that creating historical manga is rather difficult). While he's never let historical fact get completely in the way of a good story, he often does get many of the basic facts very, very right and many of his characters are based off of historial figures (Kenshin himself is based loosely on Kawakami Gensai, and Saito Hajime was lifted from history books almost wholesale). He's especially fond of the 19th century and the trappings therein, and has set most of his major works in that period.
While he was one of the true heavyweights of The Nineties, he's been somewhat overshadowed by his own students in recent years (although everyone stands in Oda's shadow these days, to some degree). Buso Renkin was decently well-received, Gun Blaze West did rather poorly, but Embalming and RK Hokkaido Arc show promise. However, a MASSIVE Role Ending Misdemeanor (namely, having been found out with underage pornography in 2018) may change all of this.