I'm not a huge fan of XY ambience recording. Sounds very boring to me - not wide enough.
I'm a big fan of ORTF for more intricate ambience recordings (although, as Nathan said, it can be a bit distracting).
My favorite single point microphone is the Sanken CSS-5. It has 3 different settings, 1 mono, 2 stereo. The normal stereo mode is 115 degrees, which is a little wider than ORTF. It sounds amazing. The second setting is a "wide" setting, at 150 degrees. The wide mode just about completely eliminates the center image, which is very useful at times.
Also a big fan of M/S for it's flexibility. You can increase or decrease the center image as necessary. It can be difficult to get an M/S rig portable though if you don't have a dedicated M/S rig. I usually use 2 AKG C414s for my M/S rig. It's not very portable, but it sounds great. I'd like to get a DPA or Gefell rig though.
A/B, Spaced Pair, Blumlein, baffled pair, Decca Tree, NOS, Binaural, etc... are all great as well. What you choose should be based on your application and the gear available to you.
There's a great resource on DPA's site on the different types of stereo recording:
For Surround Ambiance recording, you'll probably utilize some sort of holophone, or a Decca Tree, Double M/S, Fukada Tree, etc.... DPA also has a great resource on that: