With my husband and kids, it's the Monsters Inc Laugh Floor because after many years of a horrible economy in our state and us eventually having to give up everything and move to another state, we were finally able to bring our kids to DW. It was a very long, hard, emotional road that got us there, and I was so happy just to be able to do it for them. We let the kids lead us when we got into MK, and DS13 (who was 11 at the time) veered into Tomorrowland and the first ride we went on was the Laugh Floor. To see them laughing, especially DS13, who had the hardest time with our move and a new school/etc, made me cry my eyes out. When we went back, we made sure to go again, even though there really isn't anything thrilling or spectacular about the attraction - it is now our must do!
For me personally, it would be Peter Pan and Haunted Mansion. I went to DW with my grandparents, sister, and two of my aunts when I was about 7 (saw the white tiles about halfway installed on the bare round metal structure that would one day become the hallmark of EPCOT!!), and the two rides that I remember were Peter Pan and HM. I went on Peter Pan with my aunt(and godmother), and I remember being scared that I would fall out of the ship! She held onto me with one arm and pointed out all of the scenes below. I have never forgotten this...and she passed away unexpectedly at age 42 when I was 25 and my first son was 7 weeks old and her own kids were 8 and 6, and I always feel sad when I think that I was able to share DW moments with her and her own kids didn't have the chance to
HM is also a favorite - being so young the first time I went, it's hard to remember everything, but I distinctly remember sitting between my sister (who was 8) and my other aunt (who was 12) when the ghost "hitched a ride" with us at the end of the ride. I was SOOOOO excited to see this and experience it again with my own kids when they were just about the same ages!! And, it is now all three of my kids' favorite ride - we ride it at least 2-3 times each visit!