It’s arguable that radiocarbon dating more than any other scientific discovery crushed the YEC claims of a 6000 year old Earth.
I wouldn’t argue so. Radioisotope dating, perhaps. But I’d say that K-Ar dating and U-Pb dating were much more influential there.
The difficulties that YECs have with evolution pale in comparison to the difficulties they have with the laws of physics.
I would also like to nominate Phosporous-32. This isotope was invaluable in the golden age of molecular biology, from the earliest genome sequencing, immunosorbent assays, DNA/RNA interactions, and so forth. We may have moved away from using isotopes in the modern age, but they were invaluable in years gone by.