Take my words with a grain of salt as my patience with the series has its ups and downs but making it as simple as I can:
- Forget KH1. Some like, some love it, I personally played over 10 hours back when it was released but the camera, the controls and some other stuff made this interesting world a bore to explore.
- KH2 I might give it another chance myself in the future. The beginning is slow, and since I was a little skeptical I never gave it much chance. Maybe when this new HD version is released, if I can buy only KH2 on PSN I might . give it a try.
- Birth by Sleep is by far my biggest surprise. I don't even know what I had when I decided to play this when I wasn't even caring for KH anymore and I'm glad I did. The game is awesome, even with one analog less the game plays batter than KH 1 and 2 with 2 analogs. The plot is a mess is always but still enjoyable.
That's my take on the series.
I liked Dice's advice, and I think I'd stand by it. Skip 1, try 2 or Birth By Sleep, and maybe another if you happen to like it.
I personally might try KH3D someday but I think I'll wait a little more, at least when I feel like when I'm in the mood for it.