When Comet ISON was discovered and a preliminary orbit for it was worked out, it was initially announced that it could be the "comet of the century." Of course, the 21st century is only 12 years old (from 2001) and ISON turned out to be a dud. But out there in the far recesses of space there is certainly some unknown comet worthy of such an honorific title that will ultimately put on a unique and memorable show sometime during this century.
There will always be bright and spectacular comets, but in each century there is always one that will stand above the others. Below I provide my own list of the five most spectacular comets that have appeared in each century starting from the 16th and running through the 20th century. Take note that four of these five dazzlers appeared in the latter half of their century and that the average time between appearances amounts to 97 years. Considering that Comet Ikeya-Seki passed by in 1965, the next prospective "Comet of the Century" might not appear - according to our small sampling - until maybe 2029 at the earliest ... and maybe not even until the next century (in 2103!). Then again, stupendously bright comets are totally unpredictable and can suddenly appear at almost any time.
Greatest Comet of the 16th Century: The Great Comet of 1577
This comet passed to within 16.7 million miles (26.9 million kilometers) of the sun on Oct. 27, but was not sighted until five days later, when it was described in an account from Peru as an exceptionally brilliant object. Contemporary descriptions note that it was seen through the clouds like the moon. By Nov. 8, it was reported by Japanese observers as a "broom star," appearing "as bright as the moon" with a white tail spanning over 60 degrees (your clenched fist held at arm's length measures 10 degrees). The famous astronomer Tycho Brahe first saw the comet as a reflection in his garden fish pond on Nov. 13, and likened its brightness to Venus. The comet was still as bright as zero magnitude in December before it finally dropped below the limit of naked-eye visibility on Jan. 26, 1578. (Magnitude is a measure of a celestial object's brightness, with smaller numbers corresponding to brighter objects.)
A German woodcutting by Jiri Daschitzsky that depicts the Great Comet of 1577 streaking over a village of onlookers ( via mnn.com ).