Milotic has already been rejected, so you'd really have to say why this time it would be approved. Some Pokemon that were unviable in an Aegi metagame are better now, but Milotic wasn't that affected by Aegi.
Ok, maybe just a brief rundown.
Well first up I would advocate for the classic physically defensive Milotic. It is still good to use in the current OU metagame:
Milotic @ Leftovers (Physical Wall)
Ability: Competitive/ Marvel Scale
EVs: 252HP/ 252 Def/ 4 Sp. Atk
- Scald
- Ice Beam/ Toxic
- Haze/ Dragon Tail
- Recover
This is the best EV spread that Milotic should run in the OU metagame. This spread guarantees Milotic's survival against many of OU's hard-hitters and scald's burn rate helps patches up Milotic's weaker defence stat. Such a Milotic doesn't get 1 or 2HKO that easily at all by most pokemon and can proceed to toxic stall them all to death. Haze or dragon tail would be complementary to toxic as it helps prevent opposing set-ups or simply helps in phazing. However, it is noted that the usefulness of dragon tail is now curtailed due to the new fairy typing.
Competitive is also a viable ability on a defensively-spread Milotic. Most people think that Milotic runs Marvel Scale but many would be surprised by a Competitively-boosted Milotic. A +2 Milotic even with only 4 Sp. Atk EVs also deals considerable damage although it is clear that it won't be sweeping anytime soon. However, the point is to exert greater offensive pressure on the opponent whilst retaining its bulkiness. My experience is that an offensive EV spread is seldom worth it because Milotic gets blocked regardless of whether it gets full special attack investment or not; however, a defensive spread is superior because it almost always guarantees that Milotic hangs around for much longer and it's not hard to get a Competitive boost given how prevalent defog is and the occasional intimidates from gyarados or mawile. For the former, a boosted scald coupled with a burn (I know it's being a little optimistic) renders gyarados useless.
Milotic @ Leftovers (Rest Talk set)
Ability: Marvel Scale
EVs: 252HP/ 252 Def/ 4 Sp. Def
- Scald
- Dragon Tail/ Recover/ Ice Beam
- Rest
- Sleep Talk
The classic rest-talk Milotic. This EV spread guarantees that Milotic has up to 400+ physical defence. Alternatively, more may be invested into special defence such that it would be a better mixed wall. I have not tested rest talk Milotic extensively but I believe it would still work well especially with the sleep mechanics having been restored. The huge downside is the neutering of dragon tail so I do recognise that it's harder for the rest talk set to work these days.
Milotic @ Leftovers (Dedicated special wall)
Ability: Marvel Scale
EVs: 252 HP/ 88 Def/ 168 Sp Def OR 252 HP/ 252 Sp Def/ 4 Def
- Scald
- Recover
- Mirror Coat
- Ice Beam/Toxic
The former EV Spread ensures that Milotic has 216 defence which makes it slightly more durable against physical onslaughts whilst still hitting 360 sp def which is enough to tank most special hits coupled with 394 HP. However for maximum utility and in the case where you already have a dedicated physical wall then feel free to run maximum special defense. This ensures that Milotic is able to reflect back appropriate amounts of damage via mirror coat whilst allowing it to simply recover thereafter and prevent being 2HKO by exceptionally strong attackers. In any case, this set requires one to predict well and it is not entirely impossible to pull off.
Conclusion
I would really say that the first and last sets are the most viable. The rest talk set may not be that great anymore because of fairy types and the fact that Milotic only got dragon tail as a phazing move. Truth be told, Milotic is predictable as its repertoire of moves hasn't changed much over the years. But we ought to recognise the fact that Milotic is indeed resilient in that despite the minimal changes or additions to its movepool, it is not entirely obsolete and that if played right, its defensive sets can still shine. A defensive spread allows Milotic to hang around much longer and the move recover helps it retain its versatility as a turn is all that is needed for Milotic to come out afresh and ready to wall once more.
Heal bell support is appreciated to rid Milotic of nasty toxic status but it isn't mandatory of course. Milotic is usually a secondary check to many pokemon but it can serve as a dedicated counter to pokemon like heatran, infernape, entei, latios, latias, mamoswine and a slew of other ground, rock, water types etc. With marvel scale up or a lucky scald burn, fighting behemoths such as terrakion can also be taken down by Milotic. Therefore when it comes to team building, this is something the trainer must take into consideration.
I would also advocate for Competitive to be used on a defensive Milotic because that is how it can truly shine. High survivability thanks to great defenses and recover helps Milotic to inflict greater damage for a longer period of time as compared to being OHKO easily previously due to the offensive EV spread. And let's get real - Milotic isn't outspeeding anything much anytime soon with a 287 max speed with a timid nature. I would rather invest in its defenses instead.
PS: Oh and I thought it would be interesting to mention that Milotic was not at all afraid of Aegislash in the past especially with scald burning, recover and particularly so if you run haze. I played Milotic this way and it helped handle most Aegislash. Well, except the toxic stalling Aegi which is a bitch and that's why Milotic back then was run alongside earthquake Mega Venusaur which could take any of Aegi's nonsense. Further, the two form a formidable core with excellent syngery. Just wanted to point this out.