Depends on the rest of your team
Th at said, there are a are things to be kept in mind:
- Glaceon is obtained much later in game.
- Jolteon is very easy to obtai; all you need is a Thunder Stone
- Jolteon is based on Speed and Special Attack, which is much more useful in game as opposed to Glaceon's Special Defense.
- There are better Ice type Pokemon available earlier. That said, Ice type Pokemon aren't as required as Electric types in th with the game in question.
So far I'd say Jolteon, but there's one more Pokemon I'd like you to consider : Magnezone , or rather Magneton with an Eviolite. It is massively useful in game with a wider move pool and a better typing, which helps not only in the 7th gym (Steel is resistant to Dragon attacks), but in the 8th, Water type gym as well.
A good in-game move-set for Jolteon would be:
- Thunderbolt
- Shadow Ball
- Strength
- Volt Switch/ Thunder Wave
Items like Leftovers, and Expert Belt work best. Electric Gems are good for a one time use as well.
If you want to try out Magnezone, instead, a good moveset would be:
- Thunderbolt
- Flash Cannon
- Signal Beam
- Thunder Wave
Leftovers works fine fine with Magnezone.
Another alternative to this:
Magneton @ Eviolite
- Thunder Wave
- Flash Cannon
- Electro Ball
- Signal Beam/ Volt Switch
[With Higher Speed than Magnezone, and boosted defenses with Eviolite, Magneton can cause high amounts of damge with Thunder Wave to paralyze, and a highly powered Electro Ball boosted by the speed drop in the opponent.]
EDIT: Thanks to Demat, this theory is now confirmed: http://pokemondb.net/pokebase/245216/does-the-paralysis-speed-drop-affect-base-power-electro-ball
