Tactics tend to work best when tailored to the enemy, and the nice thing about the pre-battle screen in VI is it lets you do that.

One of my favorite battles was as the English desperately defending Flanders from the yearly French assaults. One year I noticed that their initial troops on the field were almost all archers or crossbows, with just a couple of spear units. I put all my cavalry out first, and while deploying my army put them as close to the attacker position as possible, along with a few swordsmen to keep the spears busy, keeping the rest of my troops way back where reinforcements could reach them quickly. When the battle started I was right next to them, and my Hobilars and Royal Knights quickly overran their whole army before they could cause any real damage. They routed en masse and I started marching back to my other forces. His cavalry caught up with my rearguard as I was pulling back, though, but they were so demoralized after the first action that they immediately routed and his whole army withdrew.

Usually, though, I play pretty conservatively with a mixed infantry battle line (mostly spears) supported by archers, with cavalry guarding the flanks. Most factions have enough unit selection to get some variation of that and it's flexible enough to pull through in most situations.