This manga is an absolute classic, and if you're planning on dabbling with any work involving Romance of the Three Kingdoms, or if you're just looking for something easy and simple to read, then this is the one for you.
This manga mainly follows Liu Bei, along with Guan Yu and Zhang Fei, during the Romance of the Three Kingdoms, but sometimes will also diverge on other important characters such as Cao Cao. The story is very simple to understand, and Yokoyama has done his absolute best to stay true to history. Of course, there are some minor details that Yokoyama has had to use his creative mind on, such as the very beginning of the manga with the Yellow Turban rebellion, as there are not enough records on those time periods to form a conclusive story. But he tells it in a way where it could very well possibly have happened during the times, so when reading through Sangokushi, it is plausible enough.
One of the biggest gripes that most manga readers have with Sangokushi is that its artwork is fairly bland and simple. However, I believe that the artwork plays along very nicely with the story. It's crisp and clean, making it very easy to understand, and it flows incredibly well with the clear and concise story. The artwork gives the manga its own charm, and I personally love it.
Now, there are two other mangas/manhaus that people associate with Sangokushi. They are Kingdom by Yasuhisa Hara and The Ravages of Time by Chan Mou. The reason they are associates is that they are tied to very important Chinese historical events, the Warring States and Romance of the Three Kingdoms respectively. However, we will be excluding Kingdom because it's about a different time period.
Sadly, there's not a lot of similarities and differences I can draw from The Ravages of Time because I simply could not make it past the first few chapters. The artwork does look nice, but it seems like it's all over the place, and the story was just too confusing at first. I know that it's based on Sima Yi's perspective, but it was extremely difficult to follow along. Not to mention, one thought I had going through my head at the time when I was trying to read The Ravages of Time was "these guys seem way too powerful". I guess it's because I've been spoiled by Sangokushi, but The Ravages of Time seems to borrow a little too much creativity. I guess it's not so bad if you treat it similarly to a shounen/seinen manga.
I guess it was my fault. I went into The Ravages of Time expecting something similar to Sangokushi, but other than the fact that they are both based upon The Romance of the Three Kingdoms, it didn't seem similar at the beginning.
And just to be clear, I do not dislike the plot of The Ravages of Time. I actually quite enjoyed it, and reading the summary was what piqued my interest in the first place. The fact that it follows Sima Yi's perspective, and actually treated Liu Bei and his brothers as the antagonists in the story, is very interesting. That was absolutely fine. What I personally had problems with was that, because the artwork was so extravagant, it also made it difficult to follow along. The story in the first few chapters were also hard to read. Because of those two points alone, I had to put the manhau down. Maybe sometime in the future, I'll pick it back up.
tl;dr Sangokushi is very basic, simple, and overall easy to read, with the simplistic artstyle making it easy on the eyes. It's also incredibly historically accurate, and keeps the main characters in line by making them act not as Shounen characters by making them perfect creatures unable to fail, but real, historical human beings that have their own ups and downs. And this is what gives Sangokushi its charm, and is what makes Sangokushi an absolute classic.