I always relish the popping up of a studio logo before watching a film - classic or contemporary. It's an unexplainable film dweeb love, but an article written yesterday by Nathaniel Rogers at The Film Experience brings it all home.
His favorites are the new Columbia logo and the all-encompassing Universal logos, but I can't believe how few people in the comments mention their favorite as the Warner Bros. logo, which is by far my favorite. I have others, of course, but nothing beats that classic Warner Bros. logo from the late 30's to the mid-40's and how great it looked before Casablanca, Saratoga Trunk, The Maltese Falcon, etc, etc.
Then there's the "what were they thinking?" logo that Warners adopted in the 70's before switching back to their classic "shield" template in 1982 and sticking with it for the duration, at least they eventually got it right.
There are some all-time moderations that I always enjoy seeing, like the RKO Radio Pictures logo, with that bleep, blee, blee, bleep sound that anyone can recognize. I also enjoy 20th Century Fox for that iconic drum roll, but at the outset of CinemaScope (around the mid-50's), that logo never looked better than it did on films like Niagara, River of No Return and The Robe.
So in summation, the classic Warner Bros. logo is king, followed by the 50's 20th Century Fox and RKO. I've always loved MGM, and then Paramount, Columbia and Universal, in that order.