I often find it hard to pick favourites, but if I were held at gunpoint and told to answer this, I think I'd go for Flounder Heights. The walls work very nicely with buckets, which is great for me; I also really like the freedom of choice regarding movement options. It's unfriendly to Chargers, I hear, but the lack of balance isn't so much of an issue for my preferences.
Still, I really like the majority of maps; whatever's present on the Ranked rotation, I feel inspired to go out there and show my mastery of the terrain (or fall flat on my face trying to do so).
I wish the poll in this thread could be updated - it's missing Alfonsino, Mahi-Mahi, Piranha and Ancho-V. Shame that the thread's poster hasn't been here for three months.