Mushoku Tensei: Isekai Ittara Honki Dasu, Souen no Historia: Shintaku Shoujo no Biblion Recommendations
Basically, both series feature the main character reincarnating into a fantasy world and becoming the most badass person out there. In mushoku tensei the main character is a fat NEET that dies in an accident and decides to live life properly after reincarnation, eventually becoming the most OP magician. In Souen no Historia, the main character is kind of a normal person who dies after saving a girl and is reincarnated into the most OP butler in existence (Think Hayate no Gotoku).
Both series feature harems and are amazing reads. If you like one you'll most likely like the other too.