Just for the fun of it, I thought I'd do up a little tier list of what I think some of the best passives in the game are and why I believe they're so powerful.
We're going to divide it into 3 categories which will be Offence, Defence, Utility, and Core(which will be passives that define the champion).
We'll things off with...
Placing as what I believe to be the third best offensive passive in the game is...
Darius with Hemorrhage.
There are two parts to this passive. First part is, of course, the bleed. Decent base damage, scales with 30% of Darius' bonus AD and stacks up to 5 times. The second part is a 5% movement speed buff for each enemy champion affect by the bleed. This is actually pretty huge. With his base speed of 340, if you have 3 enemy champions with a bleed in a team fight, you're gaining 51 bonus movement speed.
The second place winner for best offensive passive in the game is...
Kha'Zix with Unseen Threat.
This passive is a big part of Kha'Zix's early game damage. Bonus magic damage and slow on his next attack and he gets a charge of it when he leaves enemy vision. Juking in and out of brushes and using your ultimate effectively can drastically increase the total damage output of Kha'Zix
The first place winner for my opinion on best offensive passive in the game is...
Brand with Blaze.
Not only is his kit designed to combo with his passive but it is ridiculously powerful as well. 8% of a champions maximum health as damage over 4 seconds. While it is reduced my magic resistance, all it takes is a few applications of Blaze early game and your opponent is back in their base.
Moving from offensive passives, let's take a look at my top three for...
Starting with third place we have...
Xerath with Ascended Form.
With Xerath's ranges, there's not much he isn't safe from when positioned correctly. Unfortunately, a lot of attack damage bruisers can dive onto Xerath quite easily. That's where Ascended Form comes in. 15% of Xerath's ability power is added as armour. Since you're usually building Xerath pretty damn aggressive, this adds up fast.
In second place we have...
Irelia with Ionian Fervor.
While one of our favourite Ionians has been beat down with the nerf hammer on numerous occasions, that doesn't stop Ionian Fervor from making it to this list with good reason. While 10% crowd control reduction isn't much, being by 2 people ups the power to 25% reduction and if you're fighting 3 or more, you're gonna be loving the 40% reduction.
While Irelia's Ionian Fervor is amazing, there is one passive that tops all others for defence.
Poppy with Valiant Fighter.
This is an absolute nightmare of a passive. If any damage that Poppy takes exceeds 10% of her current health, it's reduced by 50%. If you deal 100 damage to a 900 health Poppy, you're now dealing 50 damage to the 900 health Poppy. Having this is rank one is pretty much a no brainer.
Anyway, as great as Valiant Fighter is, damage and survivability aren't the only things champions can have for passives. Now we move onto...
Ranking in at number three is...
Jinx with Get Excited!
I was honestly tempted to put this in with the offensive abilities but I figured it would fit in better right here. If Jinx receives a kill on a champion or tower or they died within 3 seconds of being damage by Jinx, she'll gain 175% bonus movement speed that quickly decays over 4 seconds. Repositioning, chasing, running away, you name it, this passive is perfect for such a bursty all-in marksman.
Now let's Get Excited for the number two placement.
Fizz with Nimble Fighter.
The two glorious parts of this passive are as follows. Number one, Fizz takes reduced damage from enemy auto-attacks. If you see an enemy marksman that's going mid, I'd even suggest getting a Doran's shield to stack that damage reduction. Part two allows Fizz to ignore unit collision. That alone is pretty nice but in combination with his damage reduction makes this a great passive for a melee assassin.
For our final Utility passive we have one of the best support passives in the game.
Sona with Power Chord.
After casting 3 spells, Sona will charge her next auto-attack to deal bonus damage and apply a specific effect depending on what the last spell was she cast. Hymn of Valor doubles the bonus damage, Aria of Perseverance reduces her target's damage output, and Song of Celerity slows opponents. Can't get much more utility than a passive like that which is why I believe that Power Chord is the best utility passive in the game.
Now that we've gone over all the general types of passives, we're gonna take a look at passives that define the champion they belong to. They may not always be the most powerful passives but the list toppers are great.
First passive that defines the champion it belongs to.
Morderkaiser with Iron Man.
Not only is this an awesome Black Sabbath song, but Iron Man can make Mordekaiser incredibly durable in prolonged fights. Unfortunately, not so useful when he's burst down quickly.
The second passive that truly defines it's owner's playstyle is...
Evelynn with Shadow Walk.
If this doesn't define Evelynn then I don't know what does. After a short period of time, Evelynn stealths and begins regenerating mana. Evelynn can only be detected by abilities and items that give true sight or if she gets too close to an enemy champion.
Now, there's one more passive that defines it's owner even more than Evelynn. He's a new champion but that doesn't stop his passive from being incredible.
Yasuo with Way of the Wanderer.
This passive reduces the damage of Yasuo's critical strikes but doubles his chances of getting one. All you need to buy is 50% critical strike chance and let this passive do the rest. That doesn't seem like much to define a champion though, does it? That's why Yasuo also gets a shield that charges when he moves. Any type of movement or displacement counts to charging it and, as long as he keeps moving, Yasuo can be deceptively tanky.
Well, that's it for my short just-for-fun tier list of champion passives. All I ask is that you keep in mind that this is entirely my personal opinion and I am not saying these are the absolute best passives as there are many others that are quite strong in the right situations.
Be sure to tell me what you think in a comment and happy summoning.