Well hrm...I think my favorite yarn has to be Knit Picks Panache. I really like the feel of it while it works up and it's very pretty as well. The bulky weight makes things work up quickly and easily also.
As for my least favorite yarn...probably red heart yarn of any kind. For some reason I seem to always get blisters and/or sore fingers when I knit or crochet with it. I think the acrylic is just to harsh for my skin since there are no other natural fibers mixed with it.
I am currently making a sweater from the Knit Picks Panache in the cinnamon colorway. I hope that it turns out well. I really like all the colors so I may order more in the near future!