I've decided St James's Park is mine.
It looks really amazing this time of year. I love all the weaving pathways lined by trees and those old fashioned black lamp posts, and the fact it's surrounded by huge pompous old buildings. Plus the ridiculous, intimidating, golden gates outside buc' palace. Every time I walk around there I have a bit of a 'London is awesome, isn't it?' moment. The lake was half frozen when I walked around there today, and there were some annoyed looking ducks. It also has a Pelican that eats pigeons, which makes it instantly incredible.
Primrose Hill probably comes a close second though.