I'm a sucker for Kate's. Aside from being married to one, I have yet to hear a musical Kate -- whether Bush, Nash, Taylor, or Wolf -- I haven't liked. And that's certainly true for Kate Rusby.
Kate essentially only has one trick, but it's such a deep and heartfelt one that I find myself being moved again and again. She's the prototypical English folk nightingale, and her singing is so sweet and lovely that you expect rainbows and butterflies to magically appear whenever she opens her mouth. She mines the same Trad territory as Sandy Denny, mixing in her own excellent original songs that sound centuries old, but she doesn't have Sandy's soulfulness or world weariness. But I can't really get too upset about it. Her pure Yorkshire folk soprano simply soars, and the tasteful and unobtrusive Trad accompaniment (acoustic guitars, Uillean pipes, pennywhistles, flutes, etc.) stays out of the way and puts the emphasis where it belongs -- on that remarkable voice.
Kate has nine or ten albums out now, all of them very fine. But if I had to recommend one, I'd suggest Sleepless from 1998, which features the usual Trad ballads, some unusual and welcome guest appearances from American bluegrass stalwarts Tim O'Brien and Darrell Scott, and a wondrous Iris Dement cover.
 Note: Katie's are not the same as Kate's. Katie's can be nasty, musically and personally. Katie's are the cute, pop, satanic side of Kate's. Katie's should not be mentioned in the same post as Kate's. Neither should K.T.'s, although the only one I know, Tunstall, is decent enough. Still, she's no Kate.