I use the EQ3 Max thermostats. They’re older technology than what’s around today, but work very well and reliably enough.
With openhab2, and buying new today, I’d be looking at the EQ-3 Homematic IP valves. You need to buy a controller, but you can run one on a Raspberry Pi with an antenna. They run IP over 433MHz, and that means they’re both more reliable, more battery efficient, and better at reporting current temp and valve position than the Max ones that I have. I’ve considered upgrading, but I have about 13 devices, so it’d be over a thousand (NZ) dollars to change out. I can live with what I have for a while longer.
They’re a bit hard to assemble information on as they’re quite German, but there is enough information in English to get them working. They’re also a more open standard than previously - they’re pushing Homematic IP as a broader standard that they’ve open sourced.
What I’ve deduced from what I’ve read is that Homematic came after the Max standards. Subsequent to that they brought out Homematic IP, which is basically embedding their message formats into an IP wrapper over 433MHz. There are some discussions that suggest that Homematic is better than Homematic IP, but I think those are dated as they predate the release of the new CCU3, which supports both and allows them to interoperate.