My answer would be different for different stages of my life.
As a teen: I agreed with lisinka. I was bowled over by the line created by the human body, which I assumed then to be "natural", along with the beautiful (usually romantic) music. "Fluidity" was my favorite element of ballet. Jumps and atheleticism came a distant second. Being a teen, it was Tschaikowsky I liked best. Swan Lake Act II pas de deux (all versions, especially Balanchine's) was my favorite. Firebird my second favorite.
As a young adult: Balanchine was the great teacher. I learned that what appeared natural was highly "unnatural" unless set on certain bodies, carefully and daringly trained. My fondness for certain dancers -- and disdain for others -- was at its height during this time. My definition of "classical music" expanded greatly. Leotard ballets simply staged became much more thrilling than traditional fulll-lengths with sets, costumes, etc. The sense that NYCB in the 60s and even the early 70s was at the epicenter of artistic creativity. All those artists, serious musicians and intellectuals in the lobby, night after night! And then there were all the moderns: Graham, Limon, and Humphrey being my favorites. All appeared very respectful of the connections between "modern" and the western classical tradition, esepcially in story line and the music.
Middle and advanced age: growing appreciation of the tradition behind every performance and a greater interest in the classics (to be visited, in my case, rather than deeply analysed) than before. The entire work, rather than a partisanship for individual dancers, has become more important now. The more trained the eye was, the more I saw -- and the subtler variations I could appreciate.
Starting ballet classes very late has been a revelation.

I see everything differently now, identifying so much more with what the dancers are experiencing as they move (and much greater awe for what all of them do, not only the greatest.) For the first time, I can "see" dancers making choices as they flow through all those "natural" lines that impressed me long ago.