All three are fantastic. I'd have a hard time proclaiming one "the best." Fantasy in the Sky is a tradition. Fantasmic is a show. Illuminations is an event.
But I'd choose Illuminations. Simply put, I love Epcot (that makes me biased), and seeing Illuminations is easiest, as it can be view through most of Epcot. These days, with Fantasy in the Sky showing in limited fashion, the crowds are huge, the excellent vantage points limited. And Fantasmic requires getting in the theatre.
But we find Illuminations a joy to view, and to find a viewing spot. Afterwards, we roam the park, listen to the music, and wait for the heards to thin out. For that reaons, it's aces in my book.
However, if you choose Fantasy in the Sky, avoid the heards also. I think the best time I've had in recent years was closing MK on a Saturday night. It's beautiful when it's thinned out, the lights are glowing, and the music is playing..
Pat