Here's the best pitch Michael Kopech's ever made ... the Chicago White Sox prized prospect proposed to "Riverdale" star Vanessa Morgan -- AND SHE SAID YES!!!

23-year-old Kopech -- who's rehabbin' after a Tommy John surgery last year -- was vacationing with his GF near the Grand Canyon on Wednesday ... when he got on a knee and proposed.

Morgan said yes, calling the stud pitcher afterward, "My Forever."