Using Knowledge to their advantage

Guy can buy items from his world to use in this magic world and those items give crazy buffs if eaten like the food. A nice story

Also really good, guy gets reincarnated as a Slime but he's OP as per usual and takes everyone he comes across that is cool under his wing and creates a village/ city essentially using his modern knowledge it's pretty good

Guy dies and get's sent back in time when he was still young and uses all his knowledge to absolutely dominate it's pretty good too.

Killed by God whoops lets reincarnate you with your memories intact so you can kick but with magic being easy for you. It's pretty good I think there is a LN with much more progress than this

THIS BABY RIGHT HERE IS pretty good. Guy that was a master doctor/ scientist dies of overwork and gets reincarnated with the blessing of the medicine god and can make/ delete anything he wants if he can picture it well. Wants to revolutionize the medical world of medieval times to make up for inability to save little sister from cancer in past life.

Everyone on earth got turned to stone. 2 dudes got out 1 is a super genius and is planing on remaking the modern world it's pretty good but a little rage inducing

Separate world with medieval age tech and magic invades modern day Japan. Japan goes back into their world through the portal and gives them a ROYAL ASS WHOOPIN'. There's an anime and LN I highly recommend both.