If you liked
Monster
|
...then you might like
20th Century Boys
|
Both are by Urasawa, and among his best work. Monster deals with deeper issues while 20th Century Boys focuses more on character interactions and relationships, but the basic element of good people against a terrifying, ruthless enemy is the driving force of both. Monster is more tightly plotted and cleaner in terms of unnecessary side plots and characters, so I recommend starting out with 20th Century Boys as the slightly inferior (and I hate using that word for anything Urasawa has written) work.
If you liked
Fullmetal Alchemist
|
...then you might like
Darker than Black: Kuro no Keiyakusha
|
Both deal with the rights of people who are not quite human. The Contractors and Dolls of Darker Than Black are an excellent foil to the Homunculi, and vice versa.
If you liked
Monster
|
...then you might like
Fullmetal Alchemist
|
Both feature quests for undoing what was done. For Fullmetal Alchemist, the journey is that of recovering what was lost; for the protagonist of Monster, it is to end a life mistakenly saved. Although their plots diverge a fair bit, the overall feel is similar. There is great optimism and philanthropy in both, and although FMA has more levity to it, they contain a similar balance of tragedy and happiness. They are both works of immense quality and dedication, similar in length and pacing, with an extensive cast and great characterization. 10/10 for both.
If you liked
Monster
|
...then you might like
Death Note
|
Both shows deal with similar topics: equality of human life, good vs. evil, the right of a killer to live, etc. Whereas Death Note is more concentrated on the cerebral task of investigation, Monster's focus is on morality and redemption. Both of them are top-notch, but I find Monster to be better overall.