I normally use light calvary for harassing the enemy.
I found that quite often, the enemy will try to rush out it's archers a good distance away from their infantry-line.. this is where light cav shines, to rush their archers so they aren't able to shoot (especially those f---ing annoying crossbowmen with pavise - ie. Milan, HRE, Venice)
It disturbs their formations and all that. light calvary also tends to get the f--k out of there a lot faster than Heavy Cav.. so this tactic can be repeated.

Knights are not very good unless you want to ride around and then come at the enemy from behind, or try to enclose their enemy's general . They are dreadful for hit and run tactics because they have a tendancy to keep on fighting, which usually means they get wiped out.

i guess it all depends on ur fighting style, but i tend to prefer an army of mostly Spear, halberd, pike units,, with a few units of cheap archers and expensive (esp if i can get melee capable archers) - like maybe 4 units of missle. And then heavy cavalry. I am careful to deploy my 'line' in accordance with , run...stop, engage spearwall and place on Defensive mode. Then while the enemy is engaging my infantry.. my cheap archers will lure their cav or random next units away (possibly) while the expensive ones stick close to spearmen or even engage in the back ranks.
When the timing is right, i take my heavy cavalry from where ever i've hidden them or placed and ride around and engage their infantry right in the centre from behind, splitting them into dis-array. Usually this results in all their line routing with the exception of DFK's or watever other high-morale units blinking RED - ie. fighting to the death.