This really depends.
Vitamins are probably the best if you consider only efficiency, as the increase 10 EVs with each use, but they are extremely expensive (9800 PokeDollars) and will only work for the first 100 EVs in the stat. So you can basically forget them after you get through enough EVs.
PokeRus by itself will probably take about the same amount of time as Super Training, however if it is paired with an item like one of the Power items or a Macho Brace, it will be faster than Super Training. However, battling over and over it is an incredibly boring process, so Super Training is definitely the more pleasurable of the two. PokeRus is more efficient though.
If also depends on what game you play on Super Training. The easiest will only give you four EVs, much less then what a PokeRus + item combo could give in a battle. However, the more challenging (and time consuming) games will give you twelve, which about the same or more than what PokeRus + item would give you. However, it will take you longer to get the EVs.