Paul Pierce rides with James Harden -- saying the guy is still "one of the greatest" and deserves the NBA MVP ... even after the disappointing way the Rockets ended their season.

The Truth was leaving Tao in L.A. Thursday night when he told us he'll ALWAYS support California-raised players like Harden and Russell Westbrook ... and explained why they should share the league's top individual award.

We also asked who he's rooting for in the Celtics vs. Wizards series since he played for both teams.

You can guess who he picks ...