It depends on the strategy.
The Royal Thorns strategy requires you to pick Hagrid.
The no-undead unit strategy will do well with Almaran (same as Gaudi, double crit vs demons and undead, which is the last 2 bounties and the last boss battle against Baal, the champion)
The BK/undead strategy goes well with Sirius Dark (Moro Dark in KBAP/OOTM)
A Mage seeking good intellect value will do well with Iliar (that's the name if I'm not mistaken), who is basically Elenhel from KBAP.
The rest are the same ones from KBAP + a few extra who cost more but have special abilities. The assassin companion will appear on the battlefield and strike an enemy each turn, and so on.
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