Mega Milotic
Type: Water/
Dragon
Ability: Multiscale
Base Stats: HP:95/Atk::60/Def:
119(+40)/SpA:
120(+20)/SpD:
150(+30)/Spe:
91(+10)
A large increase in it's bulk would let Milotic act as a defensive wall very effectively in OU, since it would be able to take hits well, especially Special-based. It's Water/Dragon typing is also a pretty good defensive typing and would also let it have only two weaknesses: Dragon and Fairy. Unfortunately, with the commonness of Dragon-types in the metagame, Mega Milotic may find it hard to get a spot on a team, especially over another Mega. However, what set's it appart is its ability Multiscale, allowing it to reduce damage from attacks when at full health. Pair this with Recover (or Rest) and you get a wall that is very hard to take down (Lugia can do this in Ubers since it has higher bulk, but Milotic can also do this better than Dragonite because of a better defensive-typing and bulk). It has access to Scald so it can cripple physical attackers with burns, can phaze with Dragon Tail or can reset setup sweepers with Haze. Ice Beam provides coverage, and although Mega Milotic misses out on Leftovers recovery, it makes up for it with Multiscale.
Alternatively, if Milotic gets Calm Mind in future gens, a Calm Mind set is certainly viable, boosting its SpA and SpD to very high levels. Give it Hydro Pump/Surf, Dragon Pulse and Ice Beam, Milotic hits super-effectively/neutrally against pretty much everything in OU bar Azumarill (and Empoleon, but he has nothing really to beat Milotic either). The only problem Milotic may have is the slightly inferior Speed stat, but its high bulk makes up for it and very few Special Moves would be able to touch it after setting up a few Calm Mind boosts... But it doesn't have Calm Mind (yet)
Many people may prefer to have a Water/Fairy typing for Mega Milotic instead of Water/Dragon. That would be an even better defensive typing, but being part of the Dragon Egg Group means it's pretty much impossible for that to happen. That's why I thought Water/Dragon would be the more logical option (although Water/Fairy would mirror Gyarados' Water/Dark)
So yeah, that's one of my ideas... I'm not too experienced competitively, but this was just for fun I guess
I like thinking up ideas like this so I may come up with more