The Collector of Worlds! One of the more well known villains of the Justice League, and Superman specifically. Brainiac debuted in an issue of Action Comics in 1958 with a white alien monkey companion called Koko, a character who has curiously not been a continuous part of Brainiac's mythology, almost as though it would be hard to take him seriously with a chittering ape-like Igor at his heel.
Usually portrayed as a green-skinned humanoid, flying in a giant floating metal head with tentacles, Brainiac's mostly shown as a collector, someone who wants to catalogue all existence, to the point of shrinking down cities and keeping them in his personal collection. Most notably he shrunk down Kandor, the capital city of Krypton for his collection.
While Brainiac is variously a hyper-intelligent alien, or a self-aware, incredibly sophisticated AI, his cognitive powers are basically unsurpassed in the DC Universe, allowing him to out-plan and manoeuvre his enemies, occasionally even siding with the Justice League if it's to his advantage, as is the case in the No Justice storyline.
Fun fact - the word brainiac, to mean a very intelligent person, actually originated with this character, not the other way around!