Im looking for manga where mc gets reincarnated or sent to another world and he levels up quickly. Gotta have some romance in it or atleast a small harem

Seichou Cheat de Nandemo Dekiru you ni Natta ga, Mushoku dake wa Yamerarenai you desu

~4400 manga completed
~1700 ongoing...

The New Gate
Nidome no Jinsei wo Isekai de
Dungeon Seeker

these 3 is my fav

Got some more...
Exterminator
Knights & Magic
Isekai Tensei Soudouki Gets some serious training from his mother while growing up...
Mushoku Tensei - Isekai Ittara Honki Dasu
Tsuki ga Michibiku Isekai Douchuu Though he never levels (I have read the latest translated manga chapter and if I'm not wrong he is still level 1) he IS more or less a regular person in the start... granted it's like ten pages until he is majorly OP, but he does gain more powers and stuff later on.
Kenja no Mago (does most of his leveling offscreen while growing up though)
Death March kara Hajimaru Isekai Kyousoukyoku Does most of his levelling in the first chapter, after that it is mostly gaining new skills.
Konjiki no Moji Tsukai - Yuusha Yonin ni Makikomareta Unique Cheat He starts out as "Innocent Bystander"... and in the second chapter gets magic where he can write anything and it happens. (There is somewhat of an age deficiency in his harem though... "small" harem 😁 )

Some of them might not fit exactly what you want but I've enjoyed reading all of them.

EDIT: Just felt that I should point out that if what you want is someone that grows strong, I'm not so sure The New Gate fits... as far as I know he is max level from page 1. It is a great read though.

~4400 manga completed
~1700 ongoing...

Thank you i just finished tsuki ga and i loved it

here isekais manga/manhua [url]https://www.mangaupdates.com/mylist.html?id=514068&list=user1[/url]

Thank you