I don't care for fisherman's because I don't like the way that it wears over time-- it can get very pilly and felt from use (where the edges, typically around the legs, start to felt from being exposed to wet diapers and agitation from active baby).
Little Turtle Knits yarn wears like iron-- it might start to pill, but then the pills slough off and the soaker looks brand new again. I have a LTK soaker that looks brand new and it's almost 2 years old (and it gets a lot of use). The wool also performs well in resisting moisture, and gets softer over time. It feels kind of scratchy when you knit with it though, so that's a bit of a bummer. It softens up as soon as you wash and lanolize it, and doesn't feel scratchy thereafter.
I'm one of those knitters that can only knit with yarn that I like. What I really like to knit with is Manos del uruguay yarn, which is hand spun and dyed in Uruguay by a women's cooperative. It's a blend of merino and corriedale (which, if I remember correctly, is a sheep that is 1/2 merino), it is amazingly soft and it is very lanolin rich (I don't even need to lanolize it for my heavy wetter). Makes an awesome night time cover. It's $11-14 skein, which is 3.5 oz and 138 yds. It took almost 2 full skeins to make a very (too) large soaker, but I think you'd need 2 for almost every size soaker except the very smallest. The yarn does get fuzzy (not pilly though, and the fuz is easy to remove if you want to). I love patting my son's but when he's wearing that soaker at night; it's like having an extra pet in the bed