I was just thinking this point over during the last few days, and figured it might be fun to share here, to see what each opinion might be at GAC. (I also realize it'll take a lot of thought to get there.)
Ground rules:
1) As the thread title line says, choose one Golden Age theatrical cartoon and only one that fits your choice of Best Ever. Take your time!!!!!!!
2) Only one- and two-reelers are eligible (which, for example, makes the Fleischer Popeye Specials fair game, but not any feature-length animated film from any studio...so goodbye, Snow White, and even Gulliver.)
3) And for variety's sake, you don't have to include any sort of detailed explanation of your choice (unless you truly want to)! Just have fun while you're narrowing 'em down.

To start with, here's mine:
Popeye the Sailor Meets Ali Baba's Forty Thieves (1937): Top points above all for use of color and the Fleischer tabletop technique (as we now can see without squinting, thanks to film restoration and WHV's Popeye Volume 1!), not to mention supreme fun with ad libs by all voice artists concerned. IMHO, none of those three ingredients ever came through better in the history of the Fleischer studio.
And now, the floor's open!