Milotic and gyarados?

[Luck]

It depends on the situation.
if you need a special wall, choose milotic, but if you need a physical sweeper, choose gyarados.
I like milotic more, seeing as how it has a stronger special defense and more equal stats than gyarados.
Gyarados doesn't have a good movepool, and it could be easily killed by an electric type because its speed is only average, while many electric types have a lot of speed.
Milotic is a great special wall, and can take quite a beating.
It can take care of a grass type with ice beam and can take a few hits with recover.
also note that gyarados has more weaknesses than milotic.
however, milotic is really hard to get, so getting a feeba will be hard, but rewarding.

The conclusion is that gyarados is terrible for defending purposes, and should only be used in attacking with strong moves like hydro pump.
oh, and...

Sure...milotic may be able to wall...but tell me how it would kill the opponets pokemon?

The most obvious reason...by attacking.
it only has average sp. attack, but can take a lot of hits, especially with its awesome sp. defense and restorative moves.
gyarados, however, has inhuman power but can only take 1-2 hits before its wings get clipped. Its not as fast as its hyped up to be.

 

[.]

It's still neutral against Grass, and even a 100 base Sp Def doesn't help its 4x weakness to electric, which is incredibly common. And it's weak to Stone Edge. Intimidate helps make switching in a little easier, but not much with that weakness to Stone Edge.

I'd take Milotic. Electric might as well not even be strong against it, since most people's electric attacks are special, and that Sp Def basically nullifies it.

Gyrados IS still good. Oh, and it doesn't wall, like you said. No one in their right mind would use Gyrados as a wall, because base 100 Sp Def is only good enough for an average wall, and its 4x electric weakness basically nulls that.

And? A 2x weakness in Rock can still KO it. I'd rather take a 4x weakness over all the things I mentioned.