There are many great anime series out there, so it really depends on what kind of story and genre you are in the mood for. However, some classic and highly-regarded anime series that I can recommend include:
"Fullmetal Alchemist: Brotherhood" - this is a retelling of the original "Fullmetal Alchemist" story and is considered by many to be one of the best anime series of all time. It features great characters, a compelling story, and excellent animation.
"One Piece" - is a long-running and highly popular shounen anime series. It has a huge ensemble of memorable characters, an adventurous and exciting story, and a wide range of themes, from comedy to action, drama and more.
"Death Note" - is a psychological thriller anime that tells the story of a high school student who discovers a supernatural notebook that allows him to kill anyone whose name he writes in it. The series explores themes of morality, power, and the consequences of actions.
"Attack on Titan" - is a dark fantasy anime that deals with humanity's last stand against giant humanoid creatures known as Titans. It has an intense story and great character development, as well as excellent animation and action sequences.
"Your Lie in April" - is a coming of age story that follows a former child prodigy pianist who has lost the ability to hear his music. He meets a violinist who helps him regain his passion for music. The anime is a mixture of beautiful music and emotional story.
It's also worth noting that there are many more anime series out there and these are just some of the many options available, but they are all considered as masterpieces in their genre.