I have found that trying some out is about my best bet along with reading reviews.
Karabella Aurora 8 had fabulous reviews, both for softness and for how it holds up. I bought a few skeins on sale on ebay to try out for myself, and once I knew I loved it I watched for a sale and bought up a bunch when the sale finally came. One of my very favorite to knit with! I have only just started working with it, so I can't say how it will hold up to washing, but reviews are good. It says handwash but all the reviews say it's superwash. If I could only have one merino for garments (not socks) this would be it of the ones I have tried so far.
Washing can help a lot of yarns. I have some Jaeger Matchmaker 100% merino superwash and I didn't think that much of it. I finally used it for something and it softens up very nicely when washed. Not as soft as the Karabella, but still nice.
For socks, I really like Pat Fly's yarn from Angora Valley. It blooms so nicely when washed without pilling. I toss in the washer and dryer with no problems. It's similar to Socks that Rock.
I bought some Debblie Bliss 100% Superwash Merino from littleknits sale (at $24 a bag of 10 how could I resist?). It feels soft, but I haven't tried it yet. No idea how it holds up.
Baby Ull feels very nice and has a great reputation for holding up well. But again, I don't have a lot of experience with washing it myself.