I love this question. Every time.
I'm still going with the roaring 20s because I admire the flat-chested look. (I may be the only woman in history to prefer this. I think Keira Knightley has the perfect figure). Also, there was a 'what the hell' throwing-caution-to-the-wind attitude among the youth that spilled over into the clothing trends. The shorter hemlines must have felt so freeing for women. I love the popular bobbed hairstyles. And the HEAD WEAR. Oh my. I wish we could still wear sparkly headbands with big feathers sticking out. And I still think cloche hats look charming. I also love how women wore heels that were elegant but not too high, and had t-straps to keep them on while they were doing all that exuberant swing dancing.
Second to that is the 90s, mainly for sentimental reasons. I entered adulthood in the early 90s, and it was a very happy time of life. Not every style is fab, but I liked the more relaxed, loose, casual vibe after the flamboyance of the 80s.
After that? All the eras had good and bad elements. I like a bit of everything from all of them. The 80s, for example, was the decade of big hair and ridiculous shoulder pads (boo), but also featured some awesome punk rock looks involving things like stovepipe plaid pants and cool short spiked hair.
I think the 70s is my least favourite, because of the domination of boho style, but even that decade has its pluses. I quite like flared pants, as long as they're worn with the right kind of shoes.
