A crew neck sweatshirt is the thing you put on when you want to look like you made an effort without actually making one. Cotton or a cotton blend, that ribbed neck, hem and cuffs, nothing fitted, nothing fussy. It is the jumper that does not need you to think about it.
It earns its spot from early autumn right through to the coldest part of winter, and it is good for almost everything that is not an actual occasion. Weekend errands, school runs, working from home with the heating turned down to save money, sitting about on a Sunday. Layered under a coat for the commute, it keeps you warm without the bulk of a proper jumper. Where it struggles is anything that needs you to look pulled together on purpose, a work event, a dinner where people have made an effort, a date past the third one. It reads as comfortable, which is a compliment right up until the point you need to read as polished.
Be careful what you put it with. Anything else slouchy on top, like a baggy coat or an oversized scarf, and you vanish into a shape with no waist and no point of interest. It wants one sharp thing nearby, a straight leg trouser, a slim jean, boots rather than trainers if you want it to look like a choice and not a surrender. Big statement jewellery fights the plainness of the thing rather than lifting it, so I tend to leave my neck alone and let the shape of the sweatshirt be the whole outfit.
On fit, go by the shoulder seam and the length rather than the label size. If the shoulder seam sits well down your arm and the body swamps you to mid thigh, that is not relaxed, that is just too big, and you will spend all day pulling the sleeves up to find your hands. A good one should hit around the hip, not well past it, with sleeves that stop at the wrist bone rather than halfway down your fingers. Heavier cotton holds its shape and does not go see through at the chest after a few washes, which the thin lightweight ones often do, so check the weight before you buy rather than after you have worn it to death.
It is the wrong choice whenever you need structure, a defined waist, or anything that says you tried. It will not do smart, and it will not do glamorous, and it has no interest in either. What it will do is keep you warm, look decent with almost no effort, and survive a wash at sixty without complaint. Reach for it on the days when comfort matters more than the impression you are making, which, if you are honest, is most of them.