.
Chalid can make a really tough unit like a highly promoted hero/tier 4 unit/longbowman on a hilled city reliably beatable with no losses. No other unit - not mages, not other religious heroes, can do that. Well, except for some of the Illian units, but they can't adopt religions anyway. So Chalid's the best.
Yvain can do exactly the same thing. both of them can get insanely high strength levels from their respective mana nodes making pretty much any unit "reliably beatable".