Hm...excluding PC, which is expensive in itself, I suppose it's the Vive Pro 1, which I bought 3 years (?) ago for close to 900€. That's the HMD alone, not counting Lighthouse and controllers.
And I still use it pretty often.
Intel Core i7 8700K | 32 GB DDR 4 PC 3200 | ROG STRIX Z370-F Gaming | RTX 3090 FE| Crappy Monitor| HTC Vive Pro :3