Species 8472
Species 8472 is the borg designation for an otherwise unnamed alien lifeform from another dimension called "fluidic space". The borg invaded their realm to try and assimilate them as they are supposedly the pinacle of evolution.
S-8472 is Voyager's biggest example of biowank. They have more than 100 times the DNA of humans (which in real life doesn't prove anything, certain plant species have more DNA than us). Their "bioelectric fields" wrecked havoc with Voyager's equipment. A few stray cells slapped onto Harry Kim grew to become many tenticles covering his face and upper body, slowly eating him alive. They were able to easily resist borg attempts to assimilate them through their aggressive immune system destroying the borg nanoprobes. One member of their species managed to mangle a large pile of drones by slashing them with its claws. And they can survive in the vacuum of space for an undetermined period of time.
Their ships are apparently built out of the same organic material as their bodies. They displayed very impressive abilities. They suffered little damage from borg attacks (Trektards often insist it was zero damage in spite of Tuvok commenting that a hole in a bioship was caused by a borg weapon). Their weapons easily blasted borg cubes to bits and the borg showed no sign of adapting to 8472 weapons.
Finally, a group of eight bioships, together with a special ninth ship could combine there energies creating a weapon capable of destroying entire planets. While the exact nature of the planet destroying weapon is unknown, it does not appear to operate on strict energy transfer to the target as the planet did not explode until several seconds after the termination of the beam.
They are so powerful in fact that Voyager's crew estimate that they would completely destroy the borg collective in a manner of months. Voyager's holo-doc modifies the borg nanoprobes to not be detectable by 8472 tissues in order to treat Harry and the modified nanoprobes also prove an effective weapon against the bioships. S-8472 forces subsequently retreat back to their realm after taking less than two dozen casualties--I guess with all their awesome abilities, they only want to fight when their opponent really can't fight back. Cowards.
They have appeared in Trek canon on at least two other occasions. One three-legged freak was left behind after the big retreat. The Hirogen, a hunter species decides the thing would make good prey and it is rescued by Voyager. The rest of the species decides the Federation is a threat because of the modified nanoprobes and 8472 plans on infiltrating Earth. Voyager stumbles across one of their simulation outposts in the Delta quadrant and talks them out of their plan; that diplomacy between 8472 and the Federation could be a wiser course of action.