The Stamina attribute seems to be the most important of the attributes, at least since the last MG update a few months back. In a head to head battle, Stamina almost always beats equally developed Strength, and it also seems to beat my old favorite attribute, Agility. (Note: Tommelchen - responsible for the update to the MG algorithm - also let slip a few clues at the time indicating that Stamina was going to be important.)
Overall, however, when I'm comparing my numbers with other players numbers, I take the average of the 4 primary attributes, then consider the Sentinel.
Which brings up what I think the real "secret" to the game, that the sentinel is the key - (again, since the last update.) First, while at low levels, gold is really tight, so it's difficult to buy decent weapons or armor - buying a pet is the only way to "save" gold without losing half when you sell/dismiss it, so its advised for that. But a reasonably developed pet (training at least 5-7 points per attribute when you get a new pet) will be the winning difference in Battles.
At a higher level, you may stick with a sentinel for a decent amount of time and build its points, but up until about the 10K-15K pets, probably replace your pet as soon as you make the req level/can afford the next one - leaving you with a chunk of change for bigger purchases.
Back on S5, my pet was trained well up to the 90s in its atts, and I regularly (and often badly) beat players with as much as 25-35 points higher STRENGTH, AGILITY, DEFENSE and STAMINA, but who had a far weaker Sentinel.
Just my experience, and would love to hear other recommendations!