To answer my own question...
THE Nak to start with is definitely Nang Nak (1999). It can be found with English subtitles and has the clearest story of those I've found to stream. Pee Mak, Daeng Phra Khanong and Ghost Wife all quote from it too.
Pee Mak is a must as well. It stands alone well but there's a layer of humour you'll miss if you haven't seen Nang Nak first. Daeng PK is more lazy stereotypes than actual characters and I can't recommend it, though the kids are good with what they're given and the last 8 minutes do something really great. Ghost Wife is low budget with inexperienced actors. More horror and many parallels with Nang Nak, just reset as coming of age. By the time I got to it, I was very much taken with the rhythms of the story and how much Thai film makers love Nang Nak so I was well prepared to forgive every flaw.
Also available to stream: the 1959 Mae Nak Phra Khanong, albeit with a very strange redub. The first half is their life before Mak goes to war. Mae Nak 3D also focuses on their romance and village life for the first half. It was clearly made for an audience who knows the legend well as it skims through several plot points to give more time to some of the most emotionally effective scenes I've seen in a Nak.
Of the films I've found ways to access (had really hoped to find more), I only have Mae Nak Reborn to go. There are at least two available series too, including a recent one from Ch 3, but I haven't found subtitles so I haven't taken those on yet.