Glaceon. I love ice powers and I think its design is beautiful. Though my favorite used to be Vaporeon (hence my username)
I've always liked Espeon's design. I think it's something to do with the red stone and the double-ended tail that I've admired. It's got a very simplistic design, no over-the-top-ness, yet it's also detailed enough to not look like an overly fluffy Eevee or Eevee with a different color.
I like Sylveon the most because it’s not a part of a duo/trio (yes it’s weird reasoning but let me explain). For some reason I have a hard time picking what to evolve my eevee into because I go by the “groups” (vaporeon jolteon flareon, espeon umbreon, leafeon glaceon) idk why but I first pick Sylveon first when evolving my eevee. Next favorite is vaporeon I guess because it’s a water type. Idk, I think mostly the same of the eeveelutions.
Aesthetically? Leafeon. Everything else? Vaporeon. (Though I will admit that my Sylveon is the one I use the most competitively...)