Top 3 Best Slowers

1. Frozen King Foxy: Full AoE Slow, and buffs water units damage when enemies are slowed. 
2. Frostmancer Withered Bonnie: Can get up to 45% slow depending on RNG, but even 15% is great with the stun.
3. Scarecrow Freddy: 30% AoE Slow, but nerfs your units in all stats by 5%. Bum DPS too.