It really depends. You need to deal damage to it and survive, the tactique the players made into a forever-meta is of course Turn Meter reduction. Now you need to make a TM reducing team so the Rancor attacks less, but when it comes to roster there are SO many options tricks and more you can go with.
After analysing it I came to the following squads:
Best faction for the Rancor: Clones, Rex lead.
Best synergy based tricks: Empire, Zader lead.
Best toon based tricks: Jyn and Bistan
The most effective out of those 3 is Clones BUT they are hard to farm/gear and require good potency mods.
Empire team is easy to farm but it's the least effective and also requires mods.
So I declare to you the best squad for Rancor (imo)
Jyn Erso lead, Bistan, Chaze/Wiggs and Ackbar/Cassian