Detroit Rock City is the song that got me into the Knights In Satan's Service, so I gotta say it reigns supreme -- what?  That's not what KISS stands for?  What?
I still can't help singing Beth when I'm alone in the car, but I just don't consider that one a KISS tune.  It's a Peter Criss tune.
There was a time when I listened to The Elder over and over and over and over.  Wasn't a particularly good album, but hell, I loved it.  KISS doing conceptual.  Their own The Wall.
But to list favorites, I have to mention Sloe Gin and Deuce.  Strutter too.  
And, hey, I love I Was Made for Loving You.  Absolutely hysterical.  I was quite excited to see it make its way into Moulin Rouge, by the way.
I'm glad nobody said Tears Are Falling.  Though I DID get big kick out of that Uh All Night.
Remember when Lick It Up aired for the first time ever?  A good faction of American kids (and grownups, I'm sure) were huddled around the MTV for THAT Video Premiere!  Good gracious gravy, no makeup!
- Boomtown, with cow tongue in place