As of 2013, best airline for into London, out of Paris?

How much later this year? If you are going after mid-October, BA has a sale on flights from LAX to celebrate the inauguration of the A380 service on that route. Of course you would have to get down to LA. But you could check and compare to the price from SFO to see if it is worth it.

There is no good or bad airline. There is the airline that has the most convenient flight schedule at the best price on the day you are traveling. And that can change on any given day. You should try and search on Kayak.com using the multi-city function. going: SFO - LHR
returning: CDG - SFO Whoever has the best schedule (IMO the shortest time, fewest connections) at the most reasonable price, gets my business. Non stop flights from SFO to LHR: British, United, Virgin Atlantic Non stop flights from CDG to SFO: AirFrance

"There is no good or bad airline." I beg to differ, although believing most are middling.

My husband and I have flown to London, four times in the past four years. We fly out of LAX. We have always used our son's frequent flyer miles through American Airlines. With American Airlines, you can go eithr American or British Air. We much prefer British Air. We have always been fortunate to fly business or one time, first class. We much prefer the whole experience with British Air. The seats, the wonderful food, the service and most importantly - the route We have been able to go non-stop to/from London, with British Air. If we went American Air, we had to stop in the U.S. along the way. British Air does charge higher Heathrow taxes though. We think it is worth it to go non-stop though and we very much enjoy British Air.

I forgot to mention that I believe we flew home from Paris on British Air also. Elaine

"Whoever has the best schedule (IMO the shortest time, fewest connections) at the most reasonable price, gets my business." I agree with this, with one amendment. I do find European carriers nicer than US ones, even in coach. They often use newer planes, have AVOD (audio video on demand - individual entertainment for every seat that you control), and, my new favorite feature, a USB charging port at the seat. And I definitely find the food is better. So, if you have a choice, I'd pick a European carrier. But I do agree it's not worth making a huge detour or paying a lot more just to do so.

One reason we tend to buy our tickets through the European partner in an airline alliance is that European Union law states that the airline has to pick up the bill if the flight is delayed or canceled, even by an act of nature, such as the Icelandic volcano. US carriers don't have that responsibility. During the volcano cancelations, there was a big difference what airlines did for their customers.

Three more consideratons, all fairly minor. Some airlines are more user-friendly than others in allowing advance seat selection (boo, BA). As well, European lines tend to have a slightly more interesting selection of in-flight entertainment; in other words, more European. And I always worry a little about my luggage when changing between non-partner airlines.
Mostly I agree with previous posts: Price and time are the biggies. I don't like getting up while it's still dark to catch an early departure. And "shortest time" is problematic if connections are involved. Shortest may mean a very tight connection. Instead of risking a blown connection, I will sit around for two hours. But not six hours to save a hundred dollars.