Pithovirus sibericum. Actually revived from 30,000 year old Siberian permafrost. Abstract for Pay-walled PNAS paper or New Scientist report with added viral apocalypse terror. 30% bigger than the previous biggest-known virus, the Pandoravirus, but only has five genes in common of its 500, it acts more like the Mimavirus. It uses its own machinery to replicate rather than hijacking the amoeba's genetic and protein-assembling capability.

1.5 μm by 0.5 μm, if you were wondering.