Bhragu, Vashishta etc. seems to be the first Brahmins.
Priyavrata and Uttānapāda seems to be the first Kshatriyas.
Is there any mention about the first Vaisya either in guna based or birth based varna system?
I have removed Shudra part as it's already asked in Who was the first śūdra to be born or appear on Earth? question.