Carbon pen is a common writing instrument with the characteristics of smooth ink and not easy to fade. Here are some useful carbon pens worth buying:

1. PilotV5 Hi-Tecpoint: This is a very popular carbon pen with a fine tip and smooth ink flow, suitable for daily use Writing and drawing.

2. Uni-ballSignoDX: This carbon pen is waterproof and lightfast, suitable for use outdoors or on documents that need to be stored for a long time.

3. ZebraSarasaClip: This is an affordable carbon pen with a durable casing and smooth ink flow, suitable for students and offices.

4.PentelEnerGel: This carbon pen uses special gel ink, which has a long writing time and does not require frequent replacement of refills.

5. Mitsubishi UniKuruTogaRoulette: This is a rotary carbon pen with a variety of colors to choose from for easy marking and sorting.

6. Staedtler Triplus FineLine: This carbon pen has a fine tip and smooth ink flow for detailed drawing and writing.

7. Faber-CastellGripPlus: This is an ergonomically designed carbon pen with a comfortable grip and suitable for long-term writing.

The above are some useful carbon pen recommendations. Choose the style that suits you according to your personal needs and preferences.