There are a quite a few monsters that can add amazing defensive abilities,
* very high miss chances (Will'O'the'Wisp, Displacer Beast)
* swarm type (swarmshifter, Were murder of crows/pity of doves, hellwasp swarm)
* great DR/SR/Spell Immunities (Force Golem, Iron Golem, god, ...)
* incoporeal type (ghost, Son/Daughter of Silence, Unbodied, ...)
* many are hard to kill permanently and get 2nd chances (lich, dvati, paragon, ...)
What I think is especially interesting is that usually this costs you some casting ability (Unless you do something like monster of legend, but the type limits keep most classes from doing this). So, it becomes easy to add toughness to non-casters. (Swarmshifter, Force Golem).
There are also some cases of adding it to casters ... (Will o the wisp, son/daughter of silence, unbodied), although this is mostly miss chance / incorporeality.
Thoughts?
Best,
David