How to set default shipping method in case of multiple shipping methods in magento 2?

As i understood from your question is that you want to always have a shipping method selected when someone comes on checkout page.

To achieve this, we need to override a javascript from Magento_Checkout module.

First things first, we need to create a module :

Namespace/Module/registration.php

<?php

\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'Namespace_Module',
    __DIR__
);

Namespace/Module/etc/module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Namespace_Module" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Checkout"/>
        </sequence>
    </module>
</config>

After this, we need to create the requirejs for overriding that js.

Namespace/Module/view/frontend/requirejs-config.js

var config = {
    map: {
        '*': {
            'Magento_Checkout/js/model/checkout-data-resolver': 'Namespace_Module/js/model/checkout-data-resolver'
        }
    }
};

Now we have to copy checkout-data-resolver.js from Magento_Checkout/view/frontend/web/js/model in our module with the same path Namespace_Module/view/frontend/web/js/model

After that, we need to change a condition inside the function resolveShippingRates: function (ratesData)

From:

            ...

            if (ratesData.length == 1) {
                //set shipping rate if we have only one available shipping rate
                selectShippingMethodAction(ratesData[0]);

                return;
            }

            ...

To:

           ...

            if (ratesData.length >= 1) {
                //set shipping rate if we have only one available shipping rate
                selectShippingMethodAction(ratesData[0]);

                return;
            }

            ...

And thats it, all you have to do now is:

• Activate the module : php bin/magento module:enable Namespace_Module
• Run a setup upgrade : php bin/magento setup:upgrade
• Do a static deploy : php bin/magento setup:static-content:deploy

if (ratesData.length >= 1) {
  //set shipping rate if we have only one available shipping rate
  selectShippingMethodAction(ratesData[0]);

  return;
}

This will make it impossible to select any other shipping option. This solves the issue for me:

if (ratesData.length == 1) {
    //set shipping rate if we have only one available shipping rate;
    selectShippingMethodAction(ratesData[0]);

    return;
}

if (ratesData.length >= 1) {
    //set shipping rate if we have only one available shipping rate
    if(!selectedShippingRate) {
        selectShippingMethodAction(ratesData[0]);
    }
}

There is no need for entire module to achieve this change. You can simple extend the Checkout module and override in in your theme.

1. Copy the js file checkout-data-resolver.js
   from
   vendor\magento\module-checkout\view\frontend\web\js\model
   to
   app\design\frontend\Namespace\ThemeName\Magento_Checkout\web\js\model.

2. Locate line with following code:
   if (ratesData.length == 1) {
   and replace it with:
   if (ratesData.length >= 1 && !selectedShippingRate) {