Mine has been the Emma cardigan, designed by Janine Le Cras. It is easily the best thing I have ever knit.
It started off innocently enough, with 630 yards of the lightest yarn I've ever used, Fantastic Knitting Zephyr Laceweight. The top begun with a provisional cast-on and some eyelet lace... simple enough, but once I got to the lower lace portion... well, let's just say that that's when the cursing really started.
Each row is a lace row (knitted lace as opposed to lace knitting) with no plain purl row between, and the stitch count was at way over 200 stitches, and so it took me about an hour to complete each row. It was full of p2togtbls and other such stitches... but begrudgingly, I must admit that the result is well worth the trouble. Blocked, it became the drapiest, most beautiful piece of fabric I've ever created.
By the time I got to the sleeves, I was running dangerously low on yarn. Instead of the big puffy sleeves in the pattern, I went instead with capped sleeves finished with eyelets through which I put an elastic ribbon. These were achieved by picking up the same number of stitches as in the pattern and doing the same short-row shaping without the increases between the short rows. I also only had 32 stitches between the short rows. After knitting the edging around the neck and the button band, I had 0.4 grams of the yarn left, so it's probably a good thing that I didn't go any farther with the sleeves.
My one small regret is that I didn't quite make the upper portion of the cardigan long enough. I was hoping that I'd be able to use it to modestly cover my bosom when wearing a low-cut dress, but in order to have the ribbon go under my bust, I have to rearrange my bust so that there's major cleavage going on. Oh well, it's a flaw I can live with. Everything else about the cardigan I love... The buttons are perfect, I adore the capped sleeves and that lace... oh that lace...