With the combination of Black Knight’s defense (846) and health (1458), it looks like he has the highest stat for it (Def+Health). I expanded Expired’s list and listed all other heroes with higher health, from POI’s list.
Mother North (slow) - D (819) / H (1462)
Horghall (slow) - D (650) / H (1545)
Boss Wolf (very slow) - D (826) / H (1524)
G. Owl (very slow) - D (795) / H (1483)
Given that BK’s mana generation is Average and that he has taunt (which is undispellable right?) and an improved berserker fury 
(sort of Warcry + Berserker Fury) he looks very promising as a tank!