Here in the Philippines, our common build is Black Cleave > Boots (Mr or Armor) > Full tank. This is the standard Full tank Garen build. This is fine and fun because you have one unkillable champion, provided that you would not be too greedy. It is fine but not good.
It started when I was thinking of getting a new champ on my top lane pool. I am a tank main. My mains are Tahm Kench Top, Yorick and Malphite for Blind picks. I was actually looking for a bully champ with low ban rate (one-tricking a champ with high ban rate is suicide. You would practice all day but he would be banned when you needed him.). The problem is that there are only selected champs that can bully top lane with at the same time, can be built as a tank if needed. These are called Juggernauts. Juggernauts are subclass of Fighters. These are champs that are really hard to kill, due to their kit or build, like Illiaoi, Darius, and Renekton. Accidentally, I found Garen.
Garen is not a bully champ, unless it's mid game. Early game, he has really low damage, he can be abused being a melee with no real gap closer (flash, blink, dash), and he can be kited like forever due to the issue with gap closers. However, as always, before trying a champ, I go to forums and research about the champs themselves. I checked the forums about Garen Mains and found out a strange build. They are not building Black Cleaver unless it is necessary (2+ tanks on opponent's team). This is a surprise since we are also building Darius a Black Cleave first item. They are both good on champion since Black Cleaver has "physical on-damage" effect. It means regardless of if it is an auto attack or not, it would stack as long as the damage is physical. Now here comes the two rivals. Darius, from Noxia, has a passive which makes your opponent bleed if hit by an auto attack or a damaging skill. These bleeds last for 5 seconds. They are physical damage and "dot or damage-over-time". For 5 seconds, it would inflict physical damage per second which would stack the black cleave. Garen's E also have a physical dot if the opponent stays within range during the spell. Additionally, Black cleaver also increases your move speed on damage. These conditions are perfect for the Black Cleave's effect.
However, they are building Trinity Force on them. This item has an "on-hit" effect which activates after using a skill. After a skill has been used, you will have an upgraded auto attack. This is good but the Black Cleaver is better, right? The reason I found is that the game styles between them is different. I would explain this further on the next part.