1.) Project Diva X - 7/10 - This game is pretty good, but it doesn't have very much content. Almost every song belongs to Hatsune Miku, and the interactions with the Vocaloids are limited. Which is a shame, since this is the first game to focus on their personalities.
2.) Project Diva Future Tone - 8/10 - This one has many songs, which lets every Vocaloid shine a bit. There are also more customization options. But there are no character interactions and the base gameplay gets boring pretty quickly.
3.) Project Diva F 2nd - 9/10 - This one is my favorite Project Diva game so far. There may not be as many songs as Future Tone, but there are a lot of ways to interact with the Vocaloids. Yes it can get boring, but I've spent hours spending time with the Vocaloids in this game. Even if you leave them be, they're always doing something on their own. And with the various items, you can even set alarms for yourself and play music in the diva rooms.