PAC-MAN MUSEUM+

Whilst the achievement states you need to unlock 40 tracks, the actual number of tracks you'll have unlocked for the achievement to pop will be 49 due to there being a few tracks unlocked from the get go that don't count towards this achievement. To check your progress you can use the Jukebox which lists all locked and unlocked tracks.

Obviously it's down to your own preferance how you want to go along getting this achievement, though personally if you want a quick boost of tracks to unlock then I recommend both versions of Pac-Man Arrangement (Console and Arcade). In total you can unlock 17 tracks between these 2 games alone, and again, personal preference, but I found these to be the easiest games to complete all the missions for.
There's also 1 track that is unlocked by viewing the Staff Credits in the menu.

The achievement should unlock once you've unlocked the 49th track.

Below i've listed all games and how many tracks there are to unlock in each title (excluding 1 unlocked via Staff Credits in menu):
Pac & Pal - 2
Pac-Attack - 3
Pac-In-Time - 5
Pac-Land - 2
Pac-Man - 2
Pac-Man 256 - 1
Pac-Man Arrangement (Arcade) - 8
Pac-Man Arrangement (Console) - 9
Pac-Man Battle Royale - 4
Pac-Man Championship Edition - 1
Pac-Mania - 4
Pac-Moto - 6
Pac'N'Roll Remix - 6
Super Pac-Man - 6