This one has always stood out to me, probably because it was the first Magic card I ever saw. It came as the promo for the 8ED starter kit, and the RG deck included Enormous Baloth, which also resonated with me--as it clearly did with others in this thread. It's not the most elegant flavor text, but from the get-go I interpreted this text as an translation from Elvish rather than something this guy would actually say. Those layers of meaning, more than anything else, are what drew me to Magic.
More recently, this one really cracked me up. It reads like an Arrested Development gag, if Lucille Bluth was a vampire on Dominaria.
i dig pretty much all the very old flavor texts from the Bible / Shakespeare / other famous works of literature and poetry. A lot of my other faves are already here. Big shoutouts to Feldon and Hold the Line in particular.
I like worldbuilding, larger-than-life descriptions of tragedy, and visceral edgelord power fantasies.
The seer's parables as a whole is great, and one of my favorite things about Kamigawa are all the flavor text of written letters that describe the tensions between the different people of Kamigawa, which I think does a great job of presenting the scale of the conflict.
Like Mown, I enjoy flavour text that contributes to worldbuilding for a set. The Love Song of Night and Day showed that the setting did not just contain heroes and monsters, exotic locations, and strange magic. It could also contain poetry, fable and love.
Most of my favorites have already been mentioned but this was an early one in my Magic career that stuck with me:
I generally like the feel of the 90s ones best. I think they got a bit cringey in the early 00s and never really recovered that old fantasy charm, and never took themselves with the right amount of seriousness. Some others I wanted to share though: