Formal:
* मुझे आपके ख़ास दोस्त की बहुत याद आएगी। (Mujhe aapke khaas dost ki bahut yaad aayegi.) - This literally translates to "I will miss your special friend very much."
* मैं अपने ख़ास दोस्त को बहुत याद करूँगा। (Main apne khaas dost ko bahut yaad karunga.) - This translates to "I will miss my special friend very much."
Informal:
* तुम्हारी याद बहुत आएगी। (Tumhari yaad bahut aayegi.) - This means "I will miss you very much." This can be used for a close friend.
* तुम बहुत याद आओगे। (Tum bahut yaad aaoge.) - This also means "I will miss you very much." This is a more casual way of saying it.
More emotional:
* तुम्हारी यादें हमेशा मेरे दिल में रहेंगी। (Tumhari yaadein hamesha mere dil mein rahengi.) - This translates to "Your memories will always stay in my heart." This conveys a deep sense of missing someone.
The best choice will depend on the context and your relationship with the friend.