My favorite fiber is Blue Faced Leicester wool. It's just a perfect fiber in my opinion. It's not too expensive, it has a nice staple length, it's soft, super easy to draft, takes dye well, and... well, it's just perfect. I've spun all sorts of fibers and each has it's own charm, but BFL is just lovely to work with. There's nothing fussy about it what so ever.
And ps.. my favorite supplier for undyed BFL is coppermoose.com ($16.50/pound), and for dyed BFL, spunkyhats.com $10/4oz (i think).
Least favorite fiber? Tough one... that's like asking what my least favorite knit stitch is. I love em all.
I sell most of my hand spuns, or just display them in a basket until I decide what to knit with them.