You're right, I'd forgotten that they did count as Medium. I'd just been used to seeing Durkon drawn the same size as Belkar, and assuming he was Small. The point stands though - Roy still has a significantly higher BAB, which still makes grapple checks tilted in Roy's favor.
Sure he has fight left, and can do some damage, but the OP specifically asked if that makes Durkon, "strongest revealed melee fighter in ootsverse".
And the answer to that is unequivocally NO. The comparison with Roy was to illustrate how far behind a pure melee class Durkon is, even as a vampire, and even with cleric buffs.
His power is that he's an incredibly tough cleric that can grapple solo people to death easily and can last a LONG time in combat with full heal spells. Not that he's suddenly become an unbeatable melee badass.