Unfortunately not all champs are flexible in this regard (see note at bottom), but generally speaking:
flat armor pen against low armor targets
% armor pen against high armor targets
crit against high hp targets
example:
low armor, high HP - > flat pen + crit (IE + ghostblade)
high armor, medium or low HP -> stack armor pen (cleaver + last whisper)
high armor, high HP -> fuck that guy, attack someone else
Stuff to keep in mind - >
Flat AD is often better than pen if you can't afford to build many damage items
Penetration mixed with AD is best on champions that can build full squishy
Early on most people rely on their base stats defensively. The exception would be cloth armor starts. Flat penetration is really good against champs until they start to build armor. Once you have a chain vest or more in armor, last whisper becomes extremely efficient. Late game almost every champ will have a chain vest worth of armor in addition to their base stats if there is a reasonable amount of AD on the enemy team, this is why last whisper is almost always considered a core item that should be built 3rd or 4th on damage carries, and it's why brutalizer items are generally rushed as a first or second item.
If two or more champions are building a lot of armor (100+), consider turning your brutalizer into a black cleaver. Otherwise, a ghostblade is a more efficient buy due to the active and additional flat penetration versus squishier targets.
