For instance, for years I thought New Teen Titans was my
first exposure to George Perez, only to realize some old
Avengers back issues I had were by George, so it'd probably
be the same case with JB.

The first issue I ever got that had any Byrne work was this one:

Of course that was JB only on the cover, I guess doing finishes for Al Milgrom, though I see a lot of JB here.  The inside art was Sal Buscema, another one of my favorites, but I'm sure the JB cover, plus the FF, Spider-Man AND Dr. Doom are what sold me (as it turns out only Reed is real in this issue.  Spider-Man is an impostor and all the other characters on this cover are robots working for Dr. Faustus, but, still!)

So my first issue with JB interior work was this:

I was home sick one day when I was seven.  My father brought this home for me because he was intrigued by the photo cover.  I was hooked!

So, yes, my first JB was him on the cover, nothing inside, but my first comic written and drawn by JB was the only one of his entire run with a cover he didn't draw.

(okay, I assume he drew the cover box, the eye beams and maybe the title, too, but, again, still ...)

(edited for formatting)

Still have it, and still remember why I bought it:

Alan Carter piloting a MK IX Hawk Interceptor, and the more accurate look to the Moonbase Alpha hardware compared to the earlier issues (with apologies to Joe Staton).