On my first trip I was about 9 and I was a little space cadet, so my favorites were Space Mountain and Horizons. Actually, I'd say I liked Epcot a little better overall because of all the "future-y" stuff. I was amazed by Haunted Mansion too and 20,000 Leagues was cool too, as was The Living Seas because I loved ocean themed sci-fi (sea-fi) too.
On my second trip, at about 14 years old, I loved Haunted Mansion and Space Mountain, and those are still my favorites. 20,000 Leagues was still there and I loved it too. I was, and continue to be quite enamored with The Little Mermaid show that was at the then brand-new MGM Studios.
Today, it's Haunted Mansion with a bullet. I still love Space Mountain too and anything future themed or retro-future themed. I actually love Mission: SPACE because of all the switches, and I greatly miss 20,000 Leagues and The Living Seas (Nemo's just not the same, but I still love the SeaBase). Pirates of the Caribbean is also a favorite today, but it wasn't when I was a kid.