Get a long 1h that allows you to hit something with overhead swings right in front of your horse. This is extremely handy against HA and lancers.
Always look beyond your target. And I really put the stress on this. You are not an invincible juggernaut of destruction so you always have to be sure there aren't any enemies ready to hit you after you killed the first guy on your trajectory.
Another useful advice is to never come close to archers without hiding behind buildings (or anything that doesn't reveal your presence) if they are not otherwise engaged. You can safely come very close to aware infantry (you have to stay out of reach ofc.) though.
Here is my target ranking :
1° running archers (or sometimes inf). Your inf follows them so they are engaged and hopefully can't shoot at you. You can either bump or kill the archer, anything works. This is particularly team-friendly as you stop some of your infantry teammates from being useless and spreading out. Never forget that without infantry, you are cannon-fodder.
3° unaware pikemen. It is sometimes tricky to tell apart people that intentionnally turn their back on you or are simply distracted. Anyway, when you have the opportunity to kill a pikeman, do it. Most pikemen are weak when they are helping their first line infantry teammates against your infantry. They will generally place themselves a little behind the battle line. As the pike is a very effective weapon in mass infantry fights, many pikemen tend to do this even if all the enemy cav isn't dead.
3° people attacking your archers. Easier, more useful and team-friendlier than the next category. Even better if your archer is running.
4° enemies occupied in little fights. Typically, if you see a 2v2 fight, it is a very good move to kill one of the enemies. Beware, this is sometimes hard as some people tend to facehug and turn around enemies. If you see someone (either ally or enemy or both) doing this, don't bother helping and look for something else to do. Also, be extremely cautious with 2h and polearm users. If they spot you trying to approach, they will likely try to land a hit on you when you will get close.
5° random unaware people. Generally, new players lack the awareness to spot you soon enough. This holds true for inf as well as archers, but is easier for archers as their movements are much more predictable.
The next ones are really last resort choices and much more risky.
6° aware 1h. I doubt a good 1h will ever let a 1h cav kill him up front, but the overall skill makes this 50% safe. Place your horse as far as possible from the target.
7° aware lance cav. Very hard, but possible. At least you can make them loose some time. I don't know many secrets about this and I doubt there are many.
8° aware 2h/polearms/archers/xbow/throwers. Just don't do that.
Also, it is clear that you have to mix this with the "whatever comes up first" rule. If you see an archer being chased, go help him, even if there is an unaware pikeman somewhere else. It is often better to switch target even if you fail your first attack. Once the enemy is aware of your presence, your chances are greatly reduced. You can even try to hit-and-run as many different unaware enemies as possible.
Finally, never underestimate a good o'll bump. It is often safer to simply bump the enemy and let your allies finish the job.