Is there a best guided tour service for the Louvre, Orsay, etc?
No...I enjoy Art but we did it alone. We got a museum pass at the airport and walked ahead of every line...I cant believe people would stand in them all day. The guide maps they provide was enough.
The Louvre has certified guides who provide a 90 minute introductory tour in English. Google "louvre tours" and you should get it. Edit: Paris Walks has an art historian who does tours for them.
Rent an audio guide .. they are very helpful. ParisWalks does private tours of Lourve.