I love the polished look of an I-Cord Cast On.
But, an I-Cord Cast On (link goes to picture tutorial of basic cast on for #3 below) often leaves a row of very loose stitches above the cast on. However, how you create the increases during the cast on does affect the looseness of those stitches. Here are 4 examples.
▼ Cast On #1: uses a kfb (knit into the front and back of the first stitch slipped back to the Left Needle), so:
Slip 3 back to Left Needle, kfb, k2.
▼ Cast On #2: uses a YO (yarn over) after knitting the first stitch, so:
Slip 3 back to Left Needle, k1, yo, k2.
▼ Cast On #3: uses a kbf (knit into the back then front of the first stitch slipped back to the Left Needle), so:
Slip 3 back to Left Needle, kbf, k2.
▼ Cast On #4: uses an Ryo (reverse yarn over – bring the yarn from the back of the needle, over the needle and to the back of the work; the right leg of this yo will be at the back, left leg to the front), so:
Slip 3 back to Left Needle, k1, Ryo, k2.
When slipping the Ryo back to the Left Needle, slip with the left leg in front as shown below; the Ryo will be knit through the back of the loop so you don’t even need to remove the Right Needle when slipping this stitch back to the Left Needle – simply insert the Left Needle tip to start the slip and used the Right Needle to complete the ‘knit through back of loop’.
Conclusion: For me, #4 produced the tightest cast on. As we all have our own unique way of knitting, YMMV (your mileage may vary), of course but hopefully you’ll find a variation that works for you.