It really depends on the units :
- for 'classical' unmounted melee units, I try to maintain a solid formation while being able to push aggressively, so stalwart at the center of the formation, blitz on the wings, and mobility for the flanking units (so that they can catch up the squishy targets in case of a breach or a retreat)
- for mounted units, I agree that Stalwart seems a better choice
Some example with unique units :
- for France, since the Musketeers already have high mobility, I prefer Stalwart, for they willl often go at the heart of the enemy formation
- for Rome, since the Legion already have really high defense (and high CP), I would prioritize Blitz (so that, after hurting the enemy a bit with pilum, the legion can make a sudden push, and then fortify after the advance).
- for Persia, because the Immortals have a bonus to defense and heal, but won't do marvels in offense, I prefer going full defense and using Stalwart to allow a slow and steady advance
- for Zulu... they don't need these things
- for the Danish, Berserker have more MP and have a bonus against wounded units, so using Blitz seems the best choice here (furthermore, they can heal quickly thanks to the Danish UA, in case of needs)
- for the Swedish, the Carolean have a lot of bonus (heal, offense bonus, aoe damage when killing) geared toward a sudden and decisive strike against the enemy. I might consider mobility in these cases, so that they can overrun unprepared enemy formations quickly (with some Medic among them to make so that March heals a useful amount).
etc