First off, the highest amount of EVs any pokemon can give is 3.
Pokemon that give off 3 atk EVs: Emboar, Stoutland, Gigalith, Conkeldurr, Leavanny, Krookodile, Eelektross, Haxorus, Terrakion, Tornadus, Thundurus, Zekrom.
Only Stoutland, Leavanny, Terrakion, Tornadus, Thundurus, Zekrom can be found wild
Stoutland : Route 10 / Cold storage shaking grass 5%
Leavanny: Route 6 / Lostlorn forest Shaking grass 5%
If you are talking about the highest atk stat, Zekrom First with max 438, Haxorus second with 432.