I've played all of the games except the mobile game but I try to keep up with what's going on in it. Although I will admit Coded was pretty boring to me and 358/2 days has a great story but extremely annoying gameplay, so you didn't miss much by not actually playing them. I'd have to say my favorite is a tie between KH2 and BBS out of all of them.
Yeah I go back every now and again to Play Kingdom Hearts, just to run around Traverse Town, I love fighting Maleficent, Kingdom Hearts II is by far the favorite, and Birth By Sleep and 0.2, I'm not a big fan of Flowmotion
Honestly, I love the bosses in KH2. The Organization bosses are super fun. I got the HD Remix versions of the games so I could play the FM version of KH2 for all of the data battles & Lingering Will. Haven't gotten around to it yet but I'd love to test my experience with the games as I hear they're extremely difficult. I did beat Mysterious Figure in BBSFM, and he was definitely no walk in the park. I got so frustrated with him, one time I had him down to 1HP and made one wrong move... yeah, there was a lot of anger with that one. Flowmotion was okay, it might sound cheap but I found it useful when you're on low HP to avoid the boss attacks by bouncing back and forth off the walls with it until your Curaga charges back up. And Balloonga was extremely overpowered in DDD as well.