The perhaps non-intuitive answer is - everything that reduces the need for HVAC. I'd be looking at things like passive solar design so windows are shaded in summer but let sunlight in for heat in winter. Very good insulation and building sealing. Double glazing (possibly triple if you're in a very cold climate). Thermal mass in the right places for temperature stablity.
On the HVAC side, you want a system with a high CoP and I'd probably be looking at some form of energy recovery on the ventilation side.
I live in a relatively new building in NL, where many of these configurations are applied and on its own it won’t go lower than 21C in winter and 25C in summer. Thick walls, thick anhydrous cement floor layer for insulation (although I think it’s also meant to give the option to implement floor heating), _external_ blinds on the South side.
I simply can’t justify a smart thermostat/radiator valve build, despite my inside nerd constantly whispering I should.
Only downside is the heat exchanger air recirculator: it dries the air terribly in winter. You need a humidifier to avoid getting parched skin and mucosa.
On the HVAC side, you want a system with a high CoP and I'd probably be looking at some form of energy recovery on the ventilation side.