Some nominations for mons that I have used with some frequency:
:zamazenta: A to S-
:ogerpon-wellspring: A to A+
:iron valiant: A- to A+
:raging bolt: A+ to A
:pelipper::barraskewda: A+ to B+
:ninetales-alola::ribombee: B to B+, C+ to B+
:torkoal: A to B