The earliest comic I can remember reading was Justice League of America #76, which contained reprints of two earlier issues, #7 and #12. I was only 4 or 5 years old at the time, so I couldn't really "read" it, but the strange imagery definitely left an impression. Rereading it as an adult is a slightly different experience -- Gardner Fox explains everything *to death* -- but no matter how you slice it, "The Cosmic Fun-House" is still an incredibly weird story.
(6 pages from a 25-page story)
The set-up (and believe it or not, this is the *short* version): A new amusement park has opened up in Happy Harbor. It's actually a front for a group of aliens who are kidnapping people and replacing them with lookalikes. It's all part of a plan to retrieve an alien space-probe that is due to arrive on Earth shortly. The probe contains information on the weapons of hundreds of different worlds, and you can believe these guys aren't going to use that info for anything good.
Several of the JLA members go to investigate, only to wind up captured and duplicated. The duplicates then send the rest of the team off on a wild goose chase, while they prepare to intercept the probe. The captured heroes manage to escape, and that's when things suddenly go all Grant Morrison:
Oh yeah, that's the stuff. One blast from Fox/Sekowsky in my formative years, and I was doomed to a life of comics fandom. Gosh darn it.