Macho Brace

Boosts stat gain, but lowers Speed.

Macho Brace doubles the EVs your Pokémon gains from battle, which is great for training. The Speed reduction only applies during battle while held.

This is a solid choice for EV training before you have access to Power items. Once you can get the Power Weight, Power Bracer, and similar items, those become more efficient for targeting specific stats. But Macho Brace works for everything and is easier to obtain early!