Thunderbolt: STAB, good power, chance to paralyze, high accuracy, can attack Pokemon using Fly.
Shadow Ball: Decent chance to lower Sp. Def, good power, high accuracy. Type coverage is its main use.
Signal Beam: Since Grass types are pretty much immune to electric, you want this.
Thunder Wave: A beter option than agility since it not only slows the foe down, it can stop them from attacking. This is good because very few Pokemon can outspeed Jolteon.
Other good moves for Jolteon to use are Substitute and Baton Pass.