Probably S1. Big 4 typically wants you at Senior for 2-3 years before promoting you to manager so that they can train you their way. I had 1 year as a senior when I went to Big 4 and they started me as an A2 (second year staff).
My advice is that unless you are dead set on leaving public at some point, don't go Big 4. I started off at a regional firm as well and left for Big 4 for the pay bump. I was there for 2 years before going back to the regional firm. It's very political, can be cut throat, and the hours put a major strain on my marriage. Some folks love it and are ‘built' for it, but I was not one of those.