What yarn to choose and why…
Yarn weights are, to put it mildly, a challenge! One worsted weight may not give you the same result as another. The best rule of thumb I have found is stay in the same brand. If you are using Lion brand yarn use it for the whole project, same goes for Malabrigo. Lion is thicker than Malabrigo and when i do a swatch it will invaribly give me a thicker swatch.
Sock
This yarn is ok but I find unless I plan on knitting socks it is just to skinny. I have yet to complete socks, even baby ones are a challenge for me. One is always bigger than the other…
Dk
This seems to be my go-to yarn weight. I use it for most hats, either this or worsted weight are 90% of the yarn weights I use.
Worsted
this is arguably the most popular weight of yarn, it is the one I find most often called for in patterns. I like it, its got some bulk but with colorwork I prefer DK.
A Yarn Primer
All Free Knitting has this chart on their site. Check it out along with a lot of other good resources.