I've found that for my Malamar some diversity worked very well....
Here's my current set for competitive use.
Malamar @ Muscle Band or Focus Sash
Nature: Adamant
Ability: Contrary
IVs: Speed, HP, Attack
Evs: 252 Spe, 127 Atk, 127 HP
Moveset:
Hypnosis
Topsy-turvey/ Night Slash
Psyco Cut
Superpower
While testing this set online I have discovered that an IVd speed invested Malamar will actually manage to outspeed the majority of pokemon that seem to be widely used. However, some pokemon will still outspeed no matter what due to Malamar's average speed stat. For this reason, I often set up a hypnosis first thing in the game. Due to the HP investment, you will often live the first hit if outsped, and the hypnosis then gives you several free turns to set up with Superpower as the opponent sleeps. Afterwards, you can often clean up with psyco cut or night slash even if it is resisted due to the muscle band and numerous stat boosts. I prefer to maintain Topsy-Turvey on my set when running focus sash instead of muscle band, as not only will Malamar continue to be a serious powerhouse offensively, but also this allows you to switch into a sweeper that has stat boosted (such as dragonite) and completely stop the sweep. Simply live from the focus sash and hit with topsy-turvey after.
As for other options, I have found that a choice scarfed Malamar with several PP ups used on superpower can also work well to do fast damage, as with the stat boosts that come with it you could do some serious work.
Anyways, this is just my opinion stuff... thanks for taking the time to read it if you did!