For me, everything in EPCOT is a must do. This is my favorite park, period!
We enjoy Spaceship Earth - Yes, it is dated and in need of renovation but it is a classic and we love it.
Living Seas, The Land, Soarin is the best attraction I have ever been on, Journey Into Imagination, Universe of Energy - It is so real!, TT, MS - This ride is not for everyone but I am such a Space Buff that I love it. It is as close as I will ever get to actually being an Astronaut.
I could keep on going on but it would be redundant. Why list everything in EPCOT when it has already been done in the brochures and videos? I will say this that I haven't seen anyone else mention yet. I love the Voices of Liberty at the American Adventure. I make it a point to see them every time I am there.