I wouldn't say he's the best but he is probably the most knowledgeable writer of the 20th century. The fourteenth section of Ulysses proves that. Don't care for Dubliners though. Chekov kills Joyce in the short story department.
Yeah, Dubliners were hit-or-miss. A couple of stories were excellent, but a lot of them were just plain boring. Haven't read anything by Chekov, but yeah, everybody says he's amazing.