Pear-shaped or HMS. These carabiners have a wider opening allowing more space for rope manoeuvring, e.g. using a clove or Italian hitch, or for when using double/twin ropes, e.g. trad or multi-pitching. They also come as dedicated belay carabiners featuring a little lever that locks the carabiner on the belay loop preventing cross loading. Belay carabiners are particularly useful when using belay devices that use the carabiner as integral part of the braking mechanism, e.g. any ATC or ATC-like device, Click Up and Alpine Up, etc.
D-shaped. They're the most widely used locking carabiners because of their versatility. They can be big enough not to hinder rope manoeuvrability and perfectly suitable for belaying. They're usually preferred in smaller size, thus smaller weight, as general purpose carabiners.
Oval-shaped. Oval-shaped carabiners are best used with descenders and plates, e.g. Groovei, and whenever it does not matter which way around the carabiner is used, e.g. connecting to a prusik knot when abseiling.