According to Official Guinness Records,
First isolated from a water sample collected off the Chilean coast in April 2010, Megavirus chilensis (or MGVC) has the largest capsid (protein shell) of any known virus, measuring 440 nm in diameter. This virus's total diameter is approximately 700 nm, making it larger than some bacteria, and big enough to be seen not only with an electron microscope but also with a normal light microscope. Moreover, its genome (genetic complement) is a linear, double-stranded molecule of DNA consisting of 1,259,197 base pairs, which is the largest genome so far deciphered from any virus.
For a complete list of 2010 records, please visit 2010 Guinness Records in Chile.