Vagabond doesn't have an anime, i'm not sure why, maybe because it's too explicit sometimes. There's a lot of manga that I do think they shouldn't get an adaptation, because the manga does the work and an anime wouldn't add that much (or drop too much stuff). Inoue (the author of Vagabond) is truly one of a kind, he even got the chance to see Magic Johnson because of his manga Slam Dunk (this manga alone boosted the popularity of basket in Japan). He also have a manga called Real that is still about basketball, but it's paralympic basket. The manga was on pause for so long (2014) and this year he announced that it will resume.
For the 20th Century and Monster, well both are by the same author (and only Monster have an anime of 74 episodes). I'm really not sure wich one is the best actually, 20th is more surreal if that does change anything for you. Monster is about a brain surgeon that decided to save a normal citizen instead of the mayor, causing him to lost everything he was promised (Do you save an important person or the first that arrived at the hopital ?). The thing is, he actually saved a "Monster" and he want to stop it, because he feels responsible. 20th Century is about a group of friends that when they were kids created a scenario where vilains will try to destroy the world called Book of Prophecy. Many years laters, all of the things they wrote in that book is starting to happen.
I hope I didn't say too much but that's pretty much the scenario you just need to decide wich one your prefer, because they are both so good. They are unique and well written thriller.