Slaking without Truant = uber.
After seeing how well Kyurem did in OU (that is not at all), I am not so sure about this statement.
Slaking does have a great deal of power, but its movepool is actually quite lacking, not in coverage, but in power. It only has three physical moves with higher base power than return: gunk shot, giga impact, and focus punch, all of which are terrible. For coverage it has to rely on the 75 base power punches, or goes with earthquake and hammer arm which have redundant coverage.
I personally question whether Slaking could even be in the top 20 in ou even if it lost truant.
Anyways, while Mummy makes a great partner with Slaking it basically requires that he caries pursuit, and if you want your now "uber" slaking to survive long enough to benefit your going to need a recovery move, which then limits you options of attacking.