I read that after labs tests of a number of 'vape carts and liquids' in the UK, 100% were shown to contain no delta-9 THC. Instead they were loaded up with synthetic cannabinoids or "spice".
With that in mind, none of them. Safer to get a dry herb vape. If you buy the liquids or carts then I would definitely be sending it off for lab analysis if you're bothered about what you ingest.