Given that Phantom Menace definitively and permanently put me off the franchise, it's the clear winner by 12 parsecs. But to be honest I haven't enjoyed any of the films since the original (which I saw in theaters at the age of 10*). I almost voted for Empire Strikes Back and Return of the Jedi too, but they weren't actively awful, just, seemingly more for kids under my age when they came out. (Although the friend who dragged me to see Return of the Jedi SEVEN TIMES was three years old than me, so maybe not? Maybe having to see it seven times was what killed it for me. He also dragged me to see Aliens seven times, however, and it was gripping every single time. (As for why so many repeat viewings of single films: I grew up in a rural area with few theaters...in fact the "local" theater was an hour's drive away through the woods. (Once I moved to San Francisco, I only saw one more film seven times in theaters. Much more choice in the city!)))
* I also had the Kenner action figures (the ones where you didn't actually get the action figures, but coupons for the action figures when they eventually came out!), and the radio-controlled R2-D2, and those silly inflatable lightsaber toys that deflated within minutes, and probably one or two other bits of licensed merch.