How to read this
Rankings are community opinion, not game data. Lion is the best long-term return almost everywhere: its daily digs bring Forgehammers, Truegold and charm materials. Mighty Bison is the best combat pet when you lead rallies, adding squad capacity to every rally you start. Several pets are worth raising only to the level that unlocks the next one. Ironclad War Bear is too new to have been ranked.
Rankings by spending
Level Lion and Mighty Bison first. Prerequisite pets only need the level that unlocks the next one: Moose 15 for Lion, Grizzly Bear 15 for Giant Rhino, Giant Rhino 30 for Mighty Bison. Lynx only needs level 20.
Combines the free-to-play and spender advice; no guide ranks mid spenders separately. Lion stays first; raise Mighty Bison alongside it if you lead rallies, and stop refinement at Mythic.
Mighty Bison first: more troops per march pays off even before stats catch up. Then Giant Rhino and Grizzly Bear, with Advanced Taming Marks on Rhino and Mighty Bison. The late pets are role picks; refinement to Legendary is where spending goes furthest.