So my question is, apart from electrum wallet which other wallet is the safest for long term holding of bitcoin?.
Electrum is a hot wallet, so if you are using
electrum default hot wallet, then it isn't safe for long term holding. The safest wallet for hodling Bitcoin for the long term is wallets that are offline, wallets like hardware wallets or air-gapped wallets (of which you can create with electrum).
Hot or Cold wallets are depend on people's opinions. For me, because Electrum wallet I often do use it offline, (where I can create and sign transactions offline), I assume that Electrum wallet is a kind of cold wallet, such as a hardware wallet. You can use Electrum wallet also on air gaped devices where never connected on the internet. So it's a different situation like another hot wallet such as blockchain.com, coibase wallet, or exchange wallet where if we want to use it we must connect to the internet.