Grade B- for doing the best

Compares you to all other performances on that champion in that role. Remember lots more goes into grade than KDA too (so cs/min, objective particpation, kill participation, wards placed, wards cleared, etc...)

The performance of your teammates has no direct impact on your grade, so even if you're best on the team, if every did poorly on an absolute basis, then you'd still get a butt grade.