It's a toss-up between Swish and Shine, for me. I've made several sweaters out of Shine, and I love the soft, cool, non-wool feeling of it, AND the fabulous washability. It's my go-to yarn for baby projects for those very reasons; easy to wash, and wool-free (just in case the little ones are sensitive!).
Then again, it's hard to beat Swish. It's definitely the softest superwash I've ever worked with, and it really stands up to the hype; it truly does get softer with each wash - and gets that "melty" look of a well-loved sweater.
I just ordered some Andean Silk - and I've yet to try the CotLin, so my opinion is subject to change :)
My favorite KnitPicks yarn is whatever I'm working on at the moment. I just ordered ivory Comfy (for the Fan Kimono in Vicki Square's Knit Kimono book) and iris heather Alpaca Cloud (for a Eugen Beugler diagonally-knit rectangular stole from the Knitters Magazine shawl book), so they'll be my favorites as soon as they get here.
But really, the Bare line deserves special mention. It's really great to have such a lovely assortment of ready-to-dye yarns in any weight you could ask for, superwash or non, at such great prices. Knowing that you haven't broken the bank on the yarn makes it easier to take the risk of dyeing it.
I also appreciate all of the heather colors in so many of your lines, and the muted palette in your cotton-based yarns.
Every Knit Pick yarn I have used is very special You can't find anything else out there that compares -- especially for the price.
I've used a lot of Palette for socks. Wool of the Andes Bulky knits up for distinct for a pattern that it is beautiful. I'm knitting a summer sweater and a baby blanket out of Crayon and they are really beautiful. I can't wait to try Comfy - it looks beautiful.