Ok thanks for the feedback, by Instant death I meant like strengths that are higher than toughnesses. Would you still get an invulnerable save from that? Also things like power weapons that give you instant wounds. Inv save or not?
Instant death (where the strength of a hit is twice or more the toughness of the target) doesn't deny any saves, in and of itself. If a terminator character (2+ save) got hit by a Krak Missile (St. 8, AP3), which causes instant death, he would still get a 2+ armour save against it. Basically, the only effect the Instant Death rule has is on models with more than one wound, where models that fail their save lose all their wounds. In the example above, if the character then failed his save, he would be slain outright, even if he had 3 wounds normally.
As for Invulnerable saves, they can be taken against almost anything. There are only a few examples where you cannot take an invulnerable save - and those are special rules for particular units.
They include a Callidus Assassin (with her phase knife), C'tan and necron models with a War scythe, and Demon Hunters models with certain special ammo and equipment. There might be one or two others I've missed as well, but over all the ability to ignore invulnerable saves is very very rare. This is what makes invulnerable saves so good, lots of things ignore armour, almost nothing in most armies ignores an invulnerable save.
Basically, for the most part, and invulnerable save is simply a save that you'll always get, not taken away by particular situations or common weapons like armour or cover saves.
