Formal:
* याद आएंगे (yaad aayenge) - This literally means "will be remembered" but can be used as a formal way to say "will be missed".
* बहुत याद आएंगे (bahut yaad aayenge) - This means "will be missed very much".
Informal:
* याद आएगा (yaad aayega) - This is the singular form of "yaad aayenge" and can be used for a single person.
* बहुत याद आएगा (bahut yaad aayega) - This means "will be missed very much" (singular).
* कमी खलेगी (kami khalegi) - This literally means "there will be a lack/deficiency" and is a common way to say "will be missed" in informal situations.
Examples:
* वह बहुत याद आएंगे। (vah bahut yaad aayenge.) - He will be missed very much.
* तुम हमें बहुत याद आओगे। (tum hamen bahut yaad aaoge.) - You will be missed very much by us.
* उनकी कमी खलेगी। (unki kami khalegi.) - We will miss them.
The best way to say "will be missed" in Hindi depends on the specific situation and your relationship with the person you are talking about.