I like to start classic crew-necked sweaters with a rolled ribbing neck. The doubled ribbing is cozy, warm, and stretchy. When finished, there's no tight cast-on edge to restrict movement over the head.
To start, I use invisible cast-on over a second circular needle. I then join in a round and knit several inches of double ribbing.
Next comes a single round of purl to make a fold line for the ribbing. This gives a crisper edge to the ribbing. If I wanted a more rounded edge, I would omit it.
Next, and this is the detail that makes rolled ribbing work really well, I switch the double rib pattern so that I purl where the lower section has knits and knit where the lower section has purls. When the ribbing is folded, each rib will nest in a purl valley and line up with the rib that will lie above it.
So, if I worked my initial round of ribbing * k2, p2 *, I work this second section * p2, k2 *.
I then work the second section until it's the same length as the first.
Next, I fold the ribbing on the round of purl so that the first section I knitted is on the inside. I line up the invisible cast-on stitches with the working stitches:
And knit the invisible cast-on stitches together with the working stitches.
I then continue knitting the yoke of the sweater (in this case, a round yoke):
I'll add some short rows after the colorwork in the yoke to make the back neck higher than the front neck.
I could use the same process to make a fold-over ribbed neck, like for a turtleneck. If I was doing that, I'd start with a standard cast-on and change the row of purl to a row of knit so that the the cast-on portion would fold over instead of under. I also might start with more stitches and decrease along the way to flare the fold-over portion.
There are always options in knitting.