Okay so,
This is something that changes depending on what you are looking for in a digimon game.
The most popular option is to recommend cyber sleuth, since it is something even beginners to the franchise love. It's also super accessible, being released with its sequel/side story on multiple consoles. Will warn you though, the elevator dungeon is killer

However there are other games in the franchise that deserve merit.
The ds games (digimon story, digimon story dawn/dusk) are well loved pieces of nostalgia and I lost my mind in happiness when digimon story lost evolution got a fan translation. They are very fun games to play, especially when you play both versions of dawn/dusk to see the differences such as digimon avalible, different characters talk to you and your starting home base.
The digimon world games are another kettle of fish entirely and focus on the raising sim aspect of digimon, if you grew up with a digimon device and went 'wow, why didnt they make a game based on this?' This game is for you. I've yet to play more of these games, a factor I am looking forward to changing. I liked Next Order a lot, especially putting in hard work and seeing your town progress. You also get to talk to a lot of digimon who are seen as actual individuals instead of one half of a tamer pair all the time.
If you are interested in lore, the wonderswan games are for you. They are based on the adventures of Ryo, a character that factors heavily on Ken from season 2 of digimon's backstory and makes an appearance in Digimon tamers.
Lastly if you love a good story with more tactical game play and multiple endings, Digimon survive is for you. It also deals with more darker themes such as character death.