Though I don't agree with critics who dismiss Marvel's 1960s superheroines as wimps, I've admitted in some of my ARCHETYPAL ARCHIVE essays (such as this one) that the Valkyrie was one of the few heroines whose power was close to the level of Marvel's "big guns" like Hulk, Sub-Mariner etc. But as I noted in the above cited essay, the Valkyrie begins life as a put-up-job: as an alter ego for the evil Enchantress (another one for whom I've not yet done an entry). Much later, an incredibly convoluted J.M.de Matteis story would establish that the image conjured up by the villainess was that of a real Asgardian warrior-maiden, so in a retcon sense, AVENGERS #83 in 1970 is indeed the first apperance of the Valkyrie's image, if not her essence.
The essence would appear about a year later, in INCREDIBLE HULK #142. I critiqued this story in this essay, which was originally intended to be part of a full-fledged examination of the character's myth-history. I lost interest at some point, but the HULK story remains the first time the Valkyrie takes on a decisive persona, even if it would take the aforesaid retcon to establish that she was more than just the Enchantress' spell overlaying a mortal persona.
As most Marvel readers know, the most-used version of the Valkyrie would appear about two years later, with the Valkyrie persona overlaying yet another mortal bit-player in DEFENDERS #4.