AO shouldn't even be counted though I love the atmosphere there in the modern era,so only USO and RG are in the mix. The USO is older than RG but not by much but there is no definitive answer here. It depends on where you live,honestly. A lot of Europeans,being brought up on clay,will definitely point to the RG as being second best,some even putting it in first place. And let's face it,the RG-WB period is the best time in tennis IMO,because it's in the middle of the season and everyone is usually at their best there.
Now if you are American USO will definitely be the second best for you and RG/AO don't matter much,especially RG considering that Americans don't have a fondness for clay and their relationship with the French isn't what you would call amiable. As I heard one American say,boasting that USO was the best,"RG is just another dirt court in Paris".
For me RG is the second best without a doubt and while I like the USO,it's blatant commercialism(even in this age where all slams do this because of the sponsors) makes me like it less. Which is funny considering that USO is the second oldest slam,but it doesn't feel like it at all. I think that they should do more to capture the old USO feeling there. The AO has a friendly atmosphere,RG and WB are steeped in tradition(you can feel it in the air) and the USO is like the party/commercial slam.