I just played through the game and Jolteon was honestly kind of annoying. Getting the coins for Thunderbolt is extremely annoying and, IMO, not really the most efficient use of time. That said, I opted to use the Shock Wave TM on Jolteon instead and while it wasn't terrible, 60 BP STAB did miss out on some crucial 1H or 2HKOs. Outside of electric moves, everything it learns uses the physical attack stat so that's annoying and forces it out against any grass, electric, or ground pokemon. While this is easily worked around, it's not ideal.
A pokemon that was not as annoying, but still flawed, was Starmie. There's a few downsides to it... One is that you need a Super Rod and you get it in the range of levels 15-25. By then my other pokemon were low to mid 30's. Also factor in that it has two abilities, one that is really shitty IMO and one that is decent in-game but not overly useful, so you may spend extra time catching the right one. The other is that its moves need to be taught, too... draining the Psychic TM and begging for the investment into Ice Beam (which, again, Gamecorner is not very efficient). Thunderbolt would also be nice but I think it's actually not necessary as Surf/Ice Beam/Psychic is pretty solid coverage (that E4 Slowbro and Gyarados are the only two I'd really need Tbolt for). The upside is good, though, as Starmie does well against Koga, Blaine, Giovanni and good portion of all the pokemon featured in the Elite 4 thanks to an excellent movepool. That's pretty much every major battle after removing Snorlax.