Users can quickly adapt to absolutely any situation, automatically gaining the necessary attributes to face them. In a fight, they may adapt themselves to resist abilities, replicate and enhance them to surpass their opponents, or spontaneously develop specific countermeasures. They can also analyze and understand any object, person or situation, quickly deducing the best course of action.
Users can quickly recover from any damage, develop immunities to their source, even rebuild themselves from scratch (or environmental materials/energies if fully destroyed), and are thus virtually indestructible.
The Tyrant (Akame ga Kill) has the ability to adapt to the harshest climates, conditions and attacks, even being able to evolve and stay alive when its flesh is removed and placed inside armor, making it one of the most deadly danger beasts in existence.
Zinv (Dual! Parallel Trouble Adventure) is able to adapt to anything and repair itself, the extent at which it can adapt is best shown when it survives the complete destruction of not one but two universes at the exact same time, arriving completely unscathed in a third universe.
After Alpha (Generator Rex: Heroes United) absorbed the Omega-1 nanite from Rex Salazar, he became Alpha-Omega and gained the ability to adapt and evolve against virtually anything, including copying the Ultimatrix and all the alien DNA inside of it.
Iris (Gamera) can manipulate its own chromosomes to adapt and evolve to suit its environments, as well as adapt to incorporate any DNA it absorbs...
SCP-682 - Hard-To-Destroy Reptile (SCP Foundation) has adapted to literally every conceivable way to kill it, regenerating faster than it can be killed, warping reality to survive the future being changed, even erasing it from existence has proven to make things worse by mutating it further.
The Demonkind (Steel Angel Kurumi) are able to adapt to anything/everything, thus making all forms of weaponry and attacks ineffective against them regardless of its nature.
The Terraformars (Terra Formars) are a race of evolved cockroaches who can adapt to everything in their surroundings including their enemy's powers and abilities which they pass on to the next generation...
...such as the Peacock Mantis Shrimp Terraformar who's powers were taken from Keiji Onizuka.
Cosmic Armor Superman/Thought Robot (DC Comics) can instantaneously adapt to any threat, including the plot itself trying to destroy him.
Godzilla (Shin Godzilla) can mutate his DNA at will to adapt to any situation...
...to the point of beginning to evolve into an army of human-like forms capable of combating humanity before he was frozen.
Mark X/Future Sentinel (X-Men) kills Iceman.
White Monster (Teen Titans)
Unlike other droids, Amazo (DC Comics) can adapt to any situation with the powers he replicates.