This is the 15183rd most frequent German word.
teuersten
"Teuersten" means "most expensive" in English.
In this sentence, 'teuersten' is used as a superlative adjective describing the noun 'Auto'.
Das ist das teuerste Auto, das ich je gesehen habe.
That is the most expensive car I have ever seen.
'Teuersten' is used here to form the superlative in a comparative structure with 'am' prefix.
Von all diesen Immobilien scheint diese hier am teuersten zu sein.
Of all these properties, this one seems to be the most expensive.
The word 'teuersten' modifies the noun 'Geschenke' as a superlative adjective indicating highest cost.