The spot price on unregulated exchanges which are presumably operating fractional reserves, wash trading, front running, and spoofing the order book.

And don't forget 3 billion Tethers, likely being used to manipulate margin trades. (see "the bart" pattern, which is likely an indication of either Bitmex or Bitfinex wiping out margin traders with temporary price manipulation)

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3195066

https://amycastor.com/2019/01/17/the-curious-case-of-tether-...

https://omniexplorer.info/address/1NTMakcgVwQpMdGxRQnFKyb3G1...

Trust that it's accurate and immutable. Which is backed in turn by economic incentives for miners (more accurately -- economic disincentives to trying to cheat). The former is denominated in bitcoin, and so represents the paradox of bootstrapping this system, but here we are (and this is why early adopters are rich now -- it was a seemingly insane risk to bet that those (positive) incentives would be effective)

Worldwide trust.