Which one do you like the most? The one you look out for when picking units.
Mine is regenerate. Keeps you in the fight (if you can take a hit) and doesn’t get ignored like defense can.
If your unit gets out of a fight it can recover too, by itself.
Elite (in particular on 3+ to hit units) and vicious.
These rules migitate the effects of a fluff roll, so your attacks become so much more reliable.
Phalanx. Really helps reflect a spear wall formation effectiveness at blunting a cavalry charge.
Nimble, especially on my speed 10 Fleabag Riders. “Weeeeeeeee!” around the table.
Strider, great for charging into combat through obstacles or terrain.
Brutal - who likes to lose a roll by 1 
Pathfinder. The number of times I’ve suffered for not having that rule 
Crushing Strength
… do I really need to explain? 