All in the typing.
OU uses more Scizor, Tyranitar, Latios, Salamence, Skarmory.
Judging by the typing, we see that Mega Slowbro can be decent here, but barely. Here Steelix and Aggron score over Slowbro. Keep in mind, however, Jirachi and other Steel types cannot even dent Slowbro, so yeah, the score is tied :\
Then again, Rotom W. Here, none of the above actually can take a direct Hydro Pump.
Infernape and Lucario: Both capable Mixed Sweepers. And that spells trouble for everyone, so again, we are tied here.
Starmie, Gliscor, Breloom: Here, Slowbro has a tiny advantage over the others. Very tiny.
[Ugh, why is this so hard?]
(Come to think of it, I'm sure you wont use them here, what with all the Special Sweepers.)
However, given Kyogre's prevalence, Slowbro > the other two
That's all for Uber, tbh.