I would go with Magnezone
He has the Highest BST, and has the highest of both Attack Stats, and Defense, which is really good for in game. He also has the best typing with 11 resistances, two of which being 1/4 resistance, and an immunity, it also is the only one to learn magnet rise, which is extremely helpful due to it's x4 weakness to Ground(Magnet Rise makes it so the user is immune to ground type moves for 5 turns)
Offensive STAB of Elektross: Super effective stab to Skyla's team, with x4 to Swanna, super effective STAB to Marlon's whole team, super effective to Shauntal's Driftblim, super effective to Caitlin's Sigilyph, and super effective to Iris's Archeops and Lapras.
Defensive Typing for Electross: Resistant to Elesa's whole team's Electric STAB, and to her Emolga's Flying Stab, resistant to Clay's Excadrill's Steel STAB, and Skyla's whole team's Flying STAB, as well as her Skarmory's Steel STAB, resistant to Shauntal's Driftblim's Flying STAB, and Grimsley's Bisharp's Steel STAB, and Caitlin's Sigilyph's Flying STAB, and Iris's Aggron's Steel STAB, and her Archeop's Flying STAB.
Offensive STAB for Galvantulize: Everything Elektross had and to Burgh's Swadloon and Leavanny, to Clay's Krogorock, and Grimsley's Leipard and Krookodile, as well as Caitlin's whole team except for Sigilyph, which it can handle from it's Electric STAB mentioned earlier, and finally, Iris's Hydreigon.
Defensive typing for Galvantulize: Resistant to Burgh's Grass STAB, Elesa's whole team's Electric STAB, resistant to Clay's Excadrill's Steel STAB, Skyla's Skarmory's Steel STAB, and Grimsley's Bisharp's Steel STAB, as well as his Scrafty's Fighting STAB, and Marshal's Whole Team's Fighting STAB, as well as, finally, Iris's Aggron's Steel STAB.
Offensive Typing of Magnezone: Everything Elektross has and another Archeops Super Effective STAB in Steel.
Defensive Typing of Magnezone: Resistant to Cheren's Whole Team, Immune to Roxie's Poison and resistant to her Wirlipede's Bug STAB, Resistant to Burgh's Whole Team's STAB, resistant to Elesa's Electric STAB and 1/4 to her Flying STAB, if Magnet Rise known, immune to Clay's Ground STAB, and 1/4 resistant to his Steel STAB, resistant to all of Skyla's STAB, and 1/4 to her Flying STAB, if Magnet Rise known, immune to Drayden's Ground STAB, and resistant to his Dragon STAB, resistant to Marlon's Rock STAB on his Carracosta, 1/4 resistant to Shauntal's Driftblim Flying STAB, 1/4 resistant to Grimsley's Bisharp's Steel STAB, resistant to Caitlin's Whole Team, and 1/4 resistant to her Sigilyph's Flying STAB, Resistant to Iris's Dragon STAB, and the rest of her team's STAB, except for Lapras's Water STAB, 1/4 resistances include Aggron's Steel STAB, and Archeop's Flying STAB.
Elektross has a good ability in Levitate, getting rid of it's Ground Weakness, Magnezone has good abilities in Magnet Pull, if you want to take care of a Steel Type, and a very good one in Sturdy, which allows it to survive at one HP if hit with a KOing move, as for Galvanize, Compoud eyes is pretty good as it increases the accuracy of moves, but Unnerve will probably be used very little unless your battling normal trainers, not rivals or Gyms/E4.
So, if you want a relatively good hitter, with amazing Defensive Typing and Stats, choose Magnezone it also has some pretty good abilities compared to the others, though I do admit Levitate is really good for a Ground type, but it doesn't make up for Elektross's non-dual typing that helps Magnezone so much. Plus, Magnezone line has Magnet Rise, which is just as good. It's availability is also much earlier than the other two.
Also, as mentioned above in Sinner's Answer, the magnetic field is in Chargestone Cave, a place, I'm pretty sure, right after your battle with Clay.
Sources: The pages for these three Pokemon, the GYM e4 page for BW2 and the Dual Type Chart