Thanks for asking a question on thisisyourbible.com
I would have to say Abel followed closely by Enoch. They were the first of whom it is specifically said that they prophesied:
Luke 11:50,51 that the blood of all the prophets, shed from the foundation of the world, may be required of this generation, 51 from the blood of Abel to the blood of Zechariah, who perished between the altar and the sanctuary. Yes, I tell you, it shall be required of this generation.
Jude verse 14 It was of these also that Enoch in the seventh generation from Adam prophesied....
I hope you have found this helpful.