I remember asking my father this years ago, and he said the first one was when I was 4 in 1976 and the ride was Pirates.
I've been so many times as I was growing up (at least twice a year from 1976-1985, then once in 1989, once in 1999, once in 2000 and once in 2009) that I couldn't even tell you what the first ride I remember was. My favorites were always Pirates and Haunted Mansion. Those are still my favorite rides in the MK, but ToT is now my favorite in WDW, and Space Mountain in CA is my favorite in any Disney park (only been to WDW and DLR).