1 -> CoM -> KH2 -> Days > BBS - > Coded -> 3D -> BackCover -> 0.2
1 may be hard to push through. Its controls are clunky and outdated.
CoM may feel extremely weird. Learn to use sleights or high level cards.
2 is sublime.
If you can, try and play Days. Its gameplay is pretty below average, but its script is top-notch, and it has the best lines of the series. (The movie doesn't have the greatest lines)
The rest are enjoyable enough.