Jumpers are bought on colour and worn on neckline. The neck is what decides whether a piece layers under a coat, sits over a shirt, or has to be worn alone.
Roll neck
The warmest of the three and the most formal-looking, which is a strange combination but a useful one. It works under a blazer, under a coat with the collar up, and on its own with nothing at the neck. It does not work over a collared shirt.
Crew neck
The most flexible. Wears alone, layers over a shirt with the collar out, and disappears neatly under a coat. If you are buying one jumper a year, buy this.
Quarter zip
A collar you can adjust. Zipped up it behaves like a roll neck; open it reads casual and shows whatever is underneath. Good for days that start cold and warm up.
V-neck
Built to be worn over a shirt. On its own it can look underdressed; with a collar it looks intentional.
One practical note
Cuffs and hems do the work of holding a jumper's shape. A firm rib at both ends keeps a relaxed cut from turning shapeless after a month.