I said FMA would be the best if not for Ashita no Joe, which I'd consider the best Shonen, as well as my 2nd favourite manga.
Ashita no Joe as both a series and a Shonen avoids most of the problems and cliches present within most series. It's protagonist is no ambitious boy scout or dumb muscle, but starts out as an asshole con-artist using his wit to get by before certain changes cause him to completely develop as a character. The character development in general for most of the main and some supporting characters is excellent, with it being clear how and why each character gained a new outlook on life. Moreover, this is one of the few sports series to keep stakes high, with many boxers dying or being permanently damaged due to the sport, something that Joe is forced to acknowledge and accept on many occassions, not to mention the pain he puts himself through to accomplish his goals, such as him cutting down his weight to stay in the Bantamweight class. Despite being the protagonist, his win record is also average at best, making him far from the overpowered protagonist present in most other series. It also features one of the most iconic and impactful finales of anime/manga history.
The only flaw I'd say the series has is a few of the support characters, mainly the children, gain a lot of admiration for Joe over too quick and amount of time and with little justification, though you could excuse that as kids being overly impressionable. Anime-wise, there are also a few animation errors and recycling, though for something created in the early 70s, that's easily excusable, not to mention made up for by Dezaki's brilliant directing.
Other than those minor flaws, Ashita no Joe is a series that seems to hit all the right notes; great plot twists, excellent character development, constant stakes with no plot armour and wrought with inspirational moments.
9/10