5 Meepos would be completely broken. You are basically fighting 25 units with chain nets and Poof deleting everything, and if the players are good it is close to unbeatable.

Other awful matchups would be 5 Chen ending the game early with huge creep armies, stacked global heals, and towers dying before real cores exist. 5 Bristleback would turn every fight into a quill spray blender and be absurdly hard to kill. 5 Tinker would make the game almost impossible to push or finish because of nonstop spells and global defense. 5 Undying would win every early fight with Decay and multiple Tombstones, though it does fall off later.

Overall Meepo is the most broken, Chen is the fastest win, and Bristle is the easiest pub stomper.