I'd say Lil Nik. He can end battles very fast.
Hard to beat dovekeeper, IMO. She's usually safe to plop down before any other heroes. From there, she can generally take the focus of several nearby heroes or even an entire area or two, freeing up your other heroes for a split second to attack without retaliation and build up their energy for their first proc. A revite Walla can heal her right away and then everyone can go on about their business with a huge head start on the enemy.
I'm also a big fan of Walla for the usual reasons, and find Skeletica great for taking out towers and bases while she attacks and debuffs. Stuns from magic towers are brutal in Arid Ruins, and Skelly helps there a ton.
Anybody else who can attack from a distance makes life a lot easier, too. It's nice to see difficult enemies like skelly or rockno die before you even come near their part of the map.