Can you suggest me what bitcoin wallet is the safest? and why?
I dont know Because I dont have Bitcoin

but My friend says Ledger nano is from some hardware options of existing bitcoin wallets,Ledger has been creating hardware for Bitcoin security since 2014 and most importantly strong security is offered With Ledger Nano S, secrets like private keys are never exposed: sensitive operations are isolated inside your hardware wallet within a state-of-the-art Secure Element, locked by a PIN code.