Really it depends on what role you need Milotic for. The most common set is a physically defensive set with max HP and Defence with a Bold nature. This EV spread evens up both of your defences allowing you to take hits from both sides and better check the physical fire types in the UU tier like Infernape, Darmanitan and Entei. They usually carry Dragon Tail or Haze to prevent setup sweepers, with Dragon Tail to still allow Milotic to phaze enemies out while Taunted and to rack up entry hazard damage and Haze to beat Substitute setup sweepers like SubCM Cresselia. Toxic is generally not used as Milotic will often be fishing for burns with Scald which is often the preferred status to inflict on your opponent.
But that's not necessarily the best option. You can use a more offensive spread if that's what your team needs.