Oh yeah, I have forgotten about Leech Seed- It is another one that deserves to win.
My vote is for Leech Seed obviously. It is amazing. Cool animation, a not-so-bad accuracy, awesome effect. Subseed is excellent. Destroy the opponent while you heal with their HP. Evil but at the same time, cool!
Sheer Cold is the other side is nice, with amazing animation, but it's a OHKO so it loses points. And Sludge Bomb is also an excellent move, with great animation and amazing power but Leech Seed is better IMO.
Also, I now remember. I forgot to add that is an excellent move in battle, cause you force switches, which combined with Spikes are deadly. That also can give you a free attack to boost yourself, create a Substitue or inflict some damage on the oponent. That makes some switches risky against Leech Seed. It is also an awesome counter against the Blissey (Seed and gain LOTS of HP)