This is honestly my favorite light novel ever, nevermind Zero no Tsukaima and Highschool DxD, Mushoku Tensei easily takes the cake, I'm rereading it for the 3rd time.
It's following the recent trend of reincarnation/transportation to a fantasy world, which some people are getting tired of, but this one is definitely worth the read. The MC is pretty OP, but the only reason is because of all the magical power he has, and he had to work hard to get that much, and it can be done by other residents. Another plus is that there isn't a game system where it shows his level and titles and whatnot. There is an adventurers guild with a ranking system which is a good point.
The characters are nice and fleshed out, you have a nice variety that don't get stale. The MC himself is a bit of a pervert, but he works hard to better himself in the new world and acts pretty selflessly, but isn't completely altruistic. He makes plenty of friends and allies, he also has his fair share of friends-turned-enemies and enemies-turned-friends that adds some spice to the plot. Also, slight spoiler, this is one of the few series I've seen where the MC actually makes a harem and goes all the way with them, and he has kids with all of them.
The universe itself is pretty in depth, the backstory is nice as well, the author even provided a map so you can know what the world looks like. Since the story takes place in about 22 years of the MC's life he visits almost every corner of the world so its pretty interesting.
And it has a kind of rags to riches theme that I'm particularly fond of. In the beginning he starts well off, but because of an incident he's left stranded in one of the most dangerous places in the world with nothing but his staff and traveling companions. By the end of the story he turns into a wealthy and influential figure.
This story is honestly amazing, and it definitely deserves to be called an epic, also long as hell, well worth the read.