It really depends on location. I am a huge history buff in general, so several eras appeal to me a lot. The American Civil War is interesting, but as far as American history goes, I'm more interested in blackpowder era history like the early frontier and French and Indian War; American Frontier era with the Wild West, pioneers, land rushes, the California Gold Rush, and transcontinental railroads; or the Roaring 20's and Prohibition.
Worldwide, I really like the 19th century and turn of the 20th century in general. It was an exciting time period for many places, although particularly the Americas and the British Empire. And of course, World War II.
For English history, I am a huge nerd for the Regency era, as well as Georgian and Victorian. There are also some really interesting facets to the Tudor and Edwardian periods.
For European history overall, it's gotta be 12 century. The High Middle Ages in general are awesome. Of course the Black Death is fascinating and that was more Late Middle Ages. The Renaissance is really interesting as well.
I also really enjoy the Heian and Sengoku periods of Japan, the Han Dynasty era of China, anything to do with the Silk Road, the Macedonian Greeks' rule of Egypt, the death of the British Empire in India and Africa...
I like history. D: