I think Days was the peak of story telling when it comes to characters in the KH series. I legitimately got invested in the story of Axel, Roxas and Xion and how everything unfolded with them, I get really bummed every time I pick up KHii now knowing what happens to Roxas. The game is definitely limited by the hardware it came out on and the mission based format can get super repetitive. Not to mention every single level is a scaled down retread of a level from either KH1 or 2 (which is my biggest gripe with COM and Coded as well). But I think a lot of the impact of the characters got lost in translation between the game and the cutscene compilation in the HD collection, even though the full voice acting is nice. Anyway, I hear there's a bit of a payoff in 3 when it comes to the Days trio so I'm excited for that once I get around to playing it.
As for COM, it was actually my first KH game since I didn't have a PS2. The card mechanic is intersting, I think the deck building stuff and chaining cards together was a really cool idea. The disney worlds themselves are kind of boring since, again, it's a retread the KH1 worlds but with even less plot and randomly generated, generic rooms. I think the game worked a lot better on a GBA than on the PS2. I played the PS2 remake recently and it just didn't hold any of the same charm as the GBA one. The fully animated and voice acted cutscenes are nice, but the really pretty sprite based graphics of the GBA one is just a lot more interesting than the recycled KH1 assets in the remake. And I honestly think the music and atmosphere in the original was way better. It's absolutely stellar, has a great sense of mystery and intrigue. Listening to the Castle Oblivion theme while watching the characters slowly get their memories tampered with as they head further into the heart of the castle is super spooky. I think that atmosphere was completely lost in the PS2 version. Maybe it's the way the music is so compressed on the GBA but the music really haunting, the PS2 just kind of sounds hollow.
Neither of them are perfect games by any means, they can get dull and repetitive for a lot of people, especially people who have played the first 2 numbered titles a lot because all of the locations are just rehashed, but I think they're worth taking a look at still, even if just for a little bit. As for re:coded, I thought it was fun when I played with, but it's completely forgettable.