Galactus or his starship
While its origins are ancient and were previously unknown, it has been revealed that the Ultimate Nullifier is actually an aspect of Galactus. The Nullifier resembles a small, hand-held metallic device with no apparent functionality, although it is likely similar to Galactus himself in the respect that it appears differently to different people. In fact it is the only known weapon in the universe capable of inspiring fear in Galactus.
The Nullifer appeared in the Infinity War. At Thanos's suggestion, Galactus gave it to Thanos's sometimes-rival, Quasar. Quasar was to use it in an attempt to destroy the Magus, who had acquired the collective power of 5 cosmic cubes. However, the Ultimate Nullifier's power was redirected by the incomplete Infinity Gauntlet wielded by the Magus, causing Quasar to seemingly be nullified. (Quasar was later able to escape a special afterlife for Quantum-band wearers because of his exposure to the extra-universal Star Brand.)
The Ultimate Nullifier is generally kept within Galactus' Worldship (Taa II) though at times Galactus has been known to carry it aboard his spherical starship. In the rare instances the Nullifier has been stolen or shown to be kept in other locales (such as the Fantastic Four's headquarters) later stories have shown the Nullifier to have returned to Galactus' possession, usually with no explanation. The reason behind this inconsistency may have been revealed in the Abraxas saga. In the story, Galactus demonstrated the ability to effortlessly recall the Nullifier to himself at will, even from the grasp of Abraxas, the universal embodiment of destruction.