On the whole, accuracy-lowering moves aren't useful against the Elite Four. This is because their RNG-reliant nature makes them inconsistent at dealing with opponents; often, the most efficient option is to just keep attacking. That said, three accuracy-lowering moves stand above the others.
Mud-Slap is the only move on this list to also do damage, which can be more useful than just lowering accuracy. On the other hand, it is completely useless against Lance on account of being a Ground type move, it has just 10 PP, and its 20 BP is unlikely to be particularly consequential. Diglett and Dugtrio learn Mud-Slap at Level 25.
Sand-Attack can hit Flying types and has more PP than Mud-Slap. It is also learned by more Pokemon (Pidgey, Sandshrew, Diglett, Farfetch'd, Eevee, Kabuto), making it easier to fit on your team. It does not, however, deal damage.
Smokescreen has the most PP of the three, but its distribution is limited to just the Charmander, Koffing, Horsea, and Magmar lines.
All three moves have 100% accuracy, putting them head and shoulders above Flash and Kinesis. Again, I want to stress that an accuracy-lowering move is not necessary for the Elite Four and will likely serve little purpose. That said, good luck!
