Really, really tough question. And I’m only considering the classic WB stuff, since nothing else has come close.
Sylvester and Tweety were always fun, as were the Wile E. Coyote and the Roadrunner, and Foghorn Leghorn with the “Boy Genius” deserves a mention, but can’t be seriously considered as the all-time best.
Bugs vs. Yosimite Sam was more memorable. I always liked the one where Fearless Freep didn’t show up to do his death-defying dive into the small tub of water, but the Singing Sword, Gold Rush, and climbing the Shmatterhorn also deserve a mention. Good stuff, definitely an honerable mention, but falls short only because these particular cartoons don’t quite make it into the all-time great category.
Bugs and Elmer were great, particularily in “Rabbit of Seville” and “What’s Opera, Doc?”. Two of the best cartoons ever created in my not-so-humble opinion. The pairing suffers somewhat because it’s too one-sided most of the time though - Bugs always gets the best lines and Elmer never really gets fleshed out as a character. So absolutely wonderful cartoons but not the best duo ever.
Daffy and Porky just crack me up. “Duck Dodgers” and “Robin Hood Daffy” are two of my absolute favorites. However these suffer from the same problem as the Bugs/Elmer pairing, they are much more a vehicle for Daffy with Porky clearly in second-banana mode. Some of the funniest cartoons in existence, but again a weaker interaction between the characters.
So the choice has to be Daffy and Bugs. “Duck Rabbit Duck”, “Rabbit Fire”, the Jack and the Beanstalk one (can’t remember the title), but above all “Rabbit Seasoning”. How could anybody even argue the point?