Amazing Spider-Man: Mary Jane's FIRST first appearance
From ASM #25, by Lee and Ditko.
Mary Jane was first mentioned ten issues earlier in ASM #15, as the nice young girl Aunt May wanted Peter to meet. Peter was already getting involved with Betty Brant and cringed whenever May mentioned Mary Jane's "nice personality." Yet the reveal that Mary Jane was a "jackpot" came pretty early.
We begin with Betty Brant and Liz Allen, who were Peter's Betty and Veronica before Gwen and MJ were Peter's Betty and Veronica.
And then Mary Jane makes her first full appearance in ASM #42.