What’s the simplest way to convert cryptocurrencies (Bitcoin/altcoins) to fiat and withdraw to a bank account?

This answer will needless to say vary in which country you reside and in which country you wish to cash out.

From a Exchange-to-Bank Account Conversion:

Easiest method is to convert everything to bitcoin and use the exchange you use to cash out (find out from them if they will ‘wire-transfer’ the amount to your bank account). The bank account has to be in your name. Most exchanges will not transfer money to a third party account.

If there is a local bitcoin exchange in your country, consider opening an account over there and transferring your coin(s). Sell them on the local exchange and cash out to your local bank account. the problem with this is - you’d have to go through the hassle of opening a new account. Some exchange may have a cashing our limits/restrictions (make sure your coins are under this limit).

As an added benefit, your local exchange may offer you an arbitrage opportunity, i.e. the Bitcoin might be priced higher in your local exchange, allowing you to earn a little bit on top.

Other option is to announce within your social circle or social network. These-a-days, people would be willing to pay you locally to have you buy Bitcoins, Ethereum or other alt coins for them.