Twombly naturally came up, even if he had already and most certainly if he hadn't. Then the zip guy as I call him since I can't ever remember his name and Carl is not here to ask. One of my favorites on the subject of what is art (I used to have them write papers on this subject) was the artist who painted the iceberg red. Evarisitti, the artist, also put goldfish in blenders and indicated they could turn them on ( see article link below).
Others discussed were Marla a young artist who may or may or not had help from her dad and if she did should it matter?
Then there were the 3d sidewalk artists:
My AP classes could discuss this topic almost endlessly as they could why we read literature...another topic, another day.