Trundle + Stalker's Blade
I'm not usually one to complain about this kind of thing, but it's horribly broken.
I just had a game where the enemy team had a trundle who built:

Now this doesn't really seem like a problem but considering all the factors.
- You cannot outrun him, he has a permaslow and will more often than not be faster than you.
- You cannot fight him, he has extremely high sustain and healing
What would balance Trundle: Personally the slow on his Chomp is really strong when it has such a low cooldown. (4 second base cooldown)
Why:
He already has 2 other abilities that are catered toward him catching an opponent, then Chomp is catered toward dueling them.
With the introduction of
his kit makes him near inescapable.
Passive: When an enemy unit dies Trundle gains 2-6% health Q) Chomp: On Hit - Slows the target by 75% for 0.1 second W) Frozen Domain: Grants 20-40% increased Movement Speed, 20-80% increased Attack Speed, and 8-20% increase to Healing E) Pillar of Ice: Summons a pillar at target location slowing the target area for 25-45% for 6 seconds R) Subjugate: Deals magic damage over time and steals ___% of the target's health and 40% of the target's armor and magic resist over 4 seconds and lasts 8 seconds
You can Smite Champions, Smiting a champion steals 20% of their movement speed.

- 8% movement speed
After using an ability your next basic attack gets bonus damage equal to 200% of your AD
basic attacks grant 20ms, and kills give 60ms.
Deals 8% of the target's current health as bonus physical damage
Active: Deals 10% of the target's max health as physical damage and heals based on damage dealt.
Steals 25% of the target's Movement Speed
with
a few games ago in nemesis draft, you can ult someone, then reduce the damage of the only person that can actually damage you.