It would help if you mentioned your favorite genre(Action, Romance, Comedy).
Sword Art Online https://myanimelist.net/anime/11757/Swor...
Boy becomes trapped in a online VR game along with thousands of others with the only means to escape is to beat the game which is made up of 100 levels. Any player who is killed in the game dies for real. While trapped in the game he befriends a female player and eventually has a relationship with her.
Mondaiji-tachi ga Isekai kara Kuru Sou Desu yo? https://myanimelist.net/anime/15315/Mond...
Three kids with special powers are invited to another world by a rabbit girl. She requires there help to save her failing community by having them compete in special games.
Middle school boy discovers that he is one of twelve that has a cell phone diary that can predict the future. He is put in a ninety day survival game against the eleven other dairy holders with the sole survivor becoming the new god of time and space. The only person that helps him is a girl(another dairy holder) who is in love with him to the point of insanity that she will kill anyone that means to harm him.
22 year old with a video game addiction is abducted and left on a remote island along with many others to play a real life setting of his favorite online game where he has no choice but to kill other players using special bombs in order to leave the island. He finds his online game wife(whom he never met in real life) on the island.
Overweight middle school student is invited to play a secret online game by a upper class girl. Set in the same continuity as Sword Art Online.
Toaru Majutsu no Index
In the near future there is a secret war between science and magic. High school guy who lives in a futuristic city mostly composed of schools has the power to eliminate both science and magic in his right hand. He takes on foes from both the magic and science side when there is someone in need of help.
Shakugan no Shana
Boy discovers he is not truly real like those around him after meeting a girl with fiery hair.
Strike the Blood
High school guy with a destructive power lives in a unique artificial island city where he faces numerous foes with the help of a middle school girl wielding a powerful staff.
Thousands of people find themselves trapped inside a popular online game. One person along with his guild decides to bring order and peace among the other players while trying to find a way back to there world.
Amagi Brilliant Park
Guy with a genius intellect becomes acting general manager of a amusement park on the verge of closing down. Much of the working staff are creatures from a magical realm.