I don't have one favourite, I have lots... the one I have knitted recently was the Tudora from Knitty.com. I might make another but to a looser gauge than stated as it was a bit too small for my (freakishly long) neck. Others I like include the Crofter's Cowl, Dragon Scarf, the yummy Confection and the versatile Wisp from Knitty.com.
Do you have a favourite FO?
I often wear my Prismatic scarf, knitted with the lovely skein of Manos Silk blend I received in the last swap, but my current favourite is the crochet cowl I just finished - I have taken to wearing it around the house in the evenings. Amanda has designed a similar cowl called Cassia, which I would like to try too...
Is there a style of neck warmer you particularly like?
I like the cowls for wearing with a jumper or under a jacket, although if it's really cold I might wrap a big scarf or even a shawl around my upper body under my coat to keep me cozy.