Unsurpassed means never bettered by anything since. An unsurpassed record, unsurpassed quality, remains unsurpassed.
It is a claim about the whole time since, which is why records and performances attract it. It is weaker than best, since something unsurpassed may have been equalled.
1adjectiveunsurpassedNever bettered by anything since.
“Her long jump record has remained unsurpassed for over thirty years.”
unsurpassed is a quality.
From un- + surpassed.
Etymology from Wiktionary, CC BY-SA