For maximum overall defenses, you'll always want to go 252 HP first.
With your Milotic's IVs, 252 HP / 240 Def / 16 SDef would give it the best overall defenses, assuming Marvel Scale.
Might I just ask, how come Toxic is recommended on the Sleep Talk moveset for Milotic? I mean, doesn't that halve the chance of it using a useful move while its asleep? Or is it meant to be a stall move?