In terms of 'cuteness', probably Vaporeon. Jolteon and Leafeon are too jagged to be truly 'cute'; I've never been a fan of Flareon's design, and harbour no positive feelings towards it; Espeon, Umbreon and Glaceon are certainly giving Vappy a run for its money, but I like that Vaporeon is always kinda happy in pictures, like the one in the OP. The others all look super-serious, which definitely hinders the cuteness factors of them. So, Vaporeon gets my vote.