For some players grind is one of the most interesting parts of gameplay. These players prefer to clear locations from monsters and killing bosses in the possibility of getting any loot for craft, sale or equip the character. Grinding MMO Games are only in the MMORPG genre and mostly in the fantasy and dark fantasy setting, so all games from this tag are similar and you can easily find a new project to spend some days or months in it.
Diablo - Diablo series is the best example of the Grinding MMO Games cause it is the creator of this subgenre. All projects with this name have a very similar plot, but from each new part, it is becoming better cause of the new mechanics and graphic. Nowadays with Diablo 3 and Diablo 4 Blizzards are trying to get much more casual players as possible, since the gameplay is simple and all mechanics are easy to learn and play.
Path of Exile - The best Hack&Slash project nowadays in dark fantasy setting with a huge amount of unique farm mechanics and very strong skill customisation system. As in Diablo, you can find here a lot of classes to play and a simple plot, but all other things are on the next level. Path of Exile allows you to farm and getting loot and experience in different ways, allow you to find or create your items and build a rly strong and unique character with help of the passive skill tree. It is quite a hardcore game, but if you will get enough knowledge, you will stay here for long, since there are no competitors to it right now.
LineAge 2 - It is the best example of the full MMORPG with grinding farm system. Compared to other projects where the main process is the grind, this game stands out very much due to its character development system, countless equipment items of different grades, as well as elaborated PvP content in the form of the free battles with other players and castle sieges. If you are looking for a game with tons of classes, items and monsters, and prefer slower gameplay then Hack&Slash projects can give you, then LineAge 2 will be the best choice.