As already mentioned:
Phoenicia
Draakulis
The Scourge of Honor
Reason: all of them get the most important stat boost, which is MAGIC, and therefore they do more damage which could be the difference of being able to “one shot” one level higher explore battles.
HKI is good option but the way HKI is used (mostly) is in delves, and with just 100 hoard you are close to 100% devour chance without any medaling already.
Zuul’Goth is a waste IMO.
My favourite combo right now is gold medaled Phoenicia and Gold medaled Magma Dragon. Both get nice boost to their spell damage, which already get boost from red (and brown) gems on the board.
With Maraji Queen both get 50% start and when you cast Leprechaun, both of them of them fill up a lot of times after Lep cast and you can spam T1 PvP quickly, or do E9 or maybe even E10 with just one cast from both damage dealer.