I love Guardian. For the most part, it is pretty much even for both of the teams no matter where you spawn. Worse case scenario they start camping with sniper on S3. You could just shoot up the lift, go bottom mid to green and through elbow, or just go through S1. Love it.
I've always had great fun playing BTB on Valhalla. Coming in second would have to be Orbital. SWAT is great on that map.