I've had this debate several times... actually went head to head with one of the KIIS' 80's jocks...
As far as "Kiss" in the CHR/Top 40 sense, Kiss 108 beat out KIIS by about a year or two...sort of. Although KIIS became KIIS in 1975...it became so as an AC station...it ended up flipping formats several times until 1981 when it landed back on Top 40 (prior to 1975 it was a Top 40 station, but with the calls KKDJ and without the Kiss moniker).
Kiss 108, on the other hand, started in 1979 as a Disco...and made a progression into Top 40 with the death of Disco. Now, whether or not "Disco" could have been considered "Top 40" at that point is where the "sort of" comes in. There'll never be an official answer to this, as Kiss never "flipped" to Top 40...they just progressed with the music.
I'd like to think it is Kiss 108...but what the hell do I know. That all happened before I was born. LOL