Honestly I wish I knew. The most that I know are the sets I had early on, and were possibly hand-me-downs from my older brother. The earliest I could catalog are:
I have other LEGO sets from those years (1995-1999) in System, but I don't know which I was given vs what my brother already had and I grew to play with. Even so, I had fun playing with these DUPLO sets, sometimes in tandem with System as I learned early that they were compatible.