It seems so odd i guess, because generally people don't conciously choose their favorite things (at least, i know i don't). It just sort of happens. I didn't sit down & pick ESB as my favorite movie of all time. It just IS, b/c no other movie captivates me & entertains me to the same extent. It can almost be likend to an addiction. Having seen it, i can't not think about it regularly b/c it just stuck with me as being so good. Likewise, even if i made a concious effort to like another movie more, it would just be a farce. My innate tendancy is to always return to ESB as being the movie that i gain more enjoyment from than any other. There's nothing i can do to change that.
So, the concious decision to flit from one favorite to another just seems so completely alien! Especially when that decision is made b/c of what someone else said. What others say, do, or think has almost no* bearing on my personal favorites.
So, that being said, i say don't pick your favorite based on what Lucas or anyone else says, name your favorite based on what you personally enjoy and find the most gratifying. That is, after all, the very nature of "favorite".
*I say almost no bearing b/c there is the rare circumstance where someone's external input can affect my personal enjoyment & therefore favorite. A hypothetical example: I enjoy ESB so much b/c i think the characters are real people experiencing real events. Someone shows me that, these are in fact actors in an imaginary universe,. Lightsabers aren't real. The spaceships are models. I suddenly lose a huge amount of the entertainment value i had b/c the very reason i enjoyed it has been shown to me to be false. A far fetched example, to be sure, but i think it gets the idea across.