Which Item?

Power items. They increase your EVs by 4 instead of doubling them. That's always better because the highest EVs that any Pokemon can give is 3. (Basically, the only Pokemon like that are legendaries.) Let me break it down:

Pokemon that gives 1 EV point
Power item-5 EVs (1 + 4)
Macho Brace-2 EVs (1 x 2)
5 > 2.

Pokemon that gives 2 EV points
Power item-6 EVs (2 + 4)
Macho Brace-4 EVs (2 x 2)
6 > 4.

Pokemon that gives 3 EV points
Power item-7 EVs (3 + 4)
Macho Brace-6 EVs (3 x 2)
7 > 6.
It may not seem like a big difference, but it will save you a LOT of time in the long run.