Mustrum_Ridcully
Legend
Well, the "dispell/counterspell" effect could be something very unique - more an optional thing that you put on monsters that it fits to.I don't like the idea of a creature with SR being able to disrupt a spell that affects a broad area. Imagine a Fine golem of some kind - why would it be able to disrupt a spell that would otherwise affect the whole battlefield?
Now suppose you vary the proposal a little: say the creature has to be Small or larger, and can suppress an area spell as it is being cast, but only in its own space. I would be disposed to accept that as fine. A golem on the edge of a solid fog would not have any fog in its square, and if it stepped outside the AoE the fog would fill in behind it.
But I don't think that size has to do anything with this. It doesn't matter what size a monster if, if your Mordekainen's Sword doesn't overcome its Spell Resistance, it winks out. If the monster is that small and still in a level range where you can't beat its SR or it doesn't fail your save, it is okay. It's a special effect one has to account for these monsters.
You could just pick the 2nd and 3rd one. And you can remove the "fiddly" bits of changing duration and non-damaging effects. When I go into 3.x mode, I tend to make things complicated.This is what I am talking about!
My only concern is that it is pretty fiddly, I would rather have a system that either worked the same way - all the time. Or had only two versions.

Spell Immunity could work like option 3, except the save always succeeds.