If we are talking about "attractions" in the full sense, Illuminations:ROE is by far the best for me. The layers of symbolism, music, creativity on things like the opening globe, fire that ignites when the candles light, etc. - and the true "Epcot" statement of unity and cooperation is one of the most emotional, thrilling pieces of theater I've ever seen anywhere.
If we mean it in the more traditional sense, I'll join everyone else in the Tip Top Club of the Hollywood Tower Hotel. Such an amazing piece of engineering. I also have to give mention to the Irons version of SSE (if just for the moment you turn to face Earth at the top - chills! - and the sheer grandeur of no-wand SSE itself), Haunted Mansion (minus the current graveyard and terribly placed Rapunzel's tower) and Horizons.
Disclaimer - I have not had the privilege of going to any of the international parks, so I limited this to the US parks.