The first one I came across was TEEN TITANS 13, which was reprinted in CHRISTMAS WITH THE SUPER-HEROES special in 1974. I actually got this as a Christmas present, probably because it was big and cost $1. I think this was the first time I had ever seen the Teen Titans; I may have known Aqualad from ADVENTURE COMICS or the Aquaman cartoon, but I'm sure I didn't know who Wonder Girl or Kid Flash was.
This was a great introduction to these characters for me, because this is one of the best TT adventures ever! It definitely didn't hurt that it features killer Nick Cardy art. And it definitely didn't hurt that the story was a modern "hip" re-telling of Charles Dickens' A Christmas Carol (of course). You can't argue with classics! If you have never read this story, you should. It's a truly sentimental yet exciting adventure. It's been re-printed a few times around, so you should be able to find it at an inexpensive price.
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I tracked this story down a few years after it came out as part of my hunt to get all of the JLA back issues. My copy is badly damaged on the spline and somewhat water-damaged, but I don't care. This is probably the absolute best Christmas story I have ever read. Not only does it start with Superman and Batman on their way to various orphanages to visit kids (totally in-character for both of them), but it also features all the other JLAers and what they are doing on this Christmas Eve. Of course, it's the Atlantis Festival of Lights that has me really interested; no writer ever mentioned this festival again. Who knew Aquaman was Jewish? ;-) Anyway, after a wonderful adventure where each member in turn sacrifices himself in order to save his friends, there is a wonderful "Gift of the Magi" like twist ending (you mean they aren't all dead?) and then this wonderful exchange as Black Canary, Superman, and Green Arrow try to explain Christmas to Red Tornado. The wonderful story is by Len Wein with absolutely beautiful art by the late great Dick Dillin and Dick Giordano.
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The story by Gerry Conway could have been tighter and more dramatic (the whole super-villain thing should have been pared waaay down) and the art by Dick Dillin and Frank McLaughlin is adequate with a few touches of awesomeness. But the meaning of the story: that Christmas isn't just a legend, and that good will towards men is actually a real goal for the planet, has never been done quite so well.
In fact, I liked the plot and overall story so much that I used it as the basis for one of my bilingual comics I created while I was in Japan.
Merry Christmas everybody!