i'm in your conscience
I can't say for sure which Eeveelution I used first, but I know that the first time I played Red, I evolved my Eevee into Jolteon. It wasn't on the final team (would have taken too long to catch it up with my team), but I still carried it around with me...for no reason that I can think of, honestly haha. But it's my second favorite Eeveelution, and favorite of the original three, so yeah.
As for all three, I think they're all wonderful and like Vappy, I think they're some of the best designed Pokemon in gen I. I don't think anyone can deny that they're some of the most iconic Pokemon not only from gen I, but from the franchise as a whole. And they certainly hold up today in terms of design and lovability.