King's Field was the first PSX game I played, and I still have it (big box and all). I enjoyed that game heavily, and I enjoyed King's Field 2 just as much. Same with Shadow Tower.
Bluntly, they're much easier to play if you're able to play it on an emulator. I have played through them in my RetroPie, and the speed feels *much* better. They're not easy games to get into, and they're not entirely forgiving. You can screw up and fall in a pit with no way out but to reset, for example.
I think much of the criticism you could lay on the KF series is based in the fact that it's old, slow, big, and hard. That said, the first time I saw 1:1 in Demon's Souls, I knew it was heavily influenced by KF. They didn't even try to hide it in Dark Souls.
I don't believe they're up on PSN, but I'd buy them if they were. I play through the first two each once a year or so, just to scratch that itch. It's about that time again, too.
Very interested in the answer to this question too.
My vote is on King's Field - that's KF 2 if you're importing it. It's not as big as 2, but it does have lots of things to do. Some things that you think you can't do, you can. And it's very, very cool that you can.