If I had to pick just one from the first group, probably Jolteon, but then I like all three of the first Eeveelutions for different reasons.
Espeon, hands down. I like its shiny, besides.
I always wanted to give Glaceon a proper try, but other than one or two I tried raising, never really got far with it.
More Eeveelutions seem inevitable, and it make sense flavor-wise for Eevee, but I do wish Game Freak would give other old Pokemon split evos to choose from. Not as MANY per 'mon as Eevee, though. That'd be silly...?