I don't think Downhill is that big at all. It's fairly small for a map, to be honest.
Anyhow, my favourite is Mirage. I love desert. This map is great. Hydro, meh. Grind, it's okay. Downhill is good. Mirage and Downhill are my favs. Although, the new zombie map looks interesting. I didn't play much but instead of opening that first door, I shot the roof of the elevator and it fell all the way down. :L