Shangri-la is awesome but most people won't say it's the best because they suck at it. It's probably the easiest one solo though because you can just run throughout the entire map for trains. It's super fun with a 4 man party though. There's a bridge you can try and hold up but eventually you will be overrun. It's fun to see how long you can hold it though.
Moon was waaaaay too easy.... bio-dome should have bee taken out then it would have been way more challenging.