CareerCup

It depends if the 20 test cases are consisting of E2E Test Cases+UI+Performance related then E2E , else if none of this then the test case which has most of the code coverage meaning which does touch most of the code or more % of the Functionality will be executed first.

will have to make decision based on the code changes done.
scenario whose is likely to occur more in production or may be scenarios which is having maximum code coverage

we will choose test case based on priority and risk, the test case which is most efficient in coverage in verifying affected/changed items.

The test case which should be tested is the one which can cause a "Show stopper defect" ie affecting the basic functionality of the application and stops the application abruptly without proceeding forward (has a direct impact to the customer).

For example :
For Instance - User is testing a Online Shopping site. After Selecting the required items into the Shopping Cart, he clicks on Order Button.

Expected Result : User Should be Redirected to Order and Billing Page

Actual Result : Application displays a webServer error and displays a http Page not found error page.

This would be logged as a ShowStopper bug since you do not have any workaround to proceed ahead with testing the Orders Page.

rg

Let us assume, there are 20 test cases but I will filter the functional test cases. Because functional scenarios contains more code (it will covers majority of the code if we do code coverage analysis). Let us consider, there are 13 functional test cases but again I will filter the functional test cases based on the essential or important features like Login and Sign Up. Out of 13 functional test cases, I will check for these essential features then I will pick 1 test case which contains these essential features to be executed first.

Provided I know the functionality coverage in each test case from 20 Test cases. I will execute the one which is more close to end user scenario.
Example : Application Under Test is a flight booking application.
Single Test case : Lunch application and book a flight ticket.
User should be able to launch application, search flight for desired destination and should be able to book flight successfully.

I would pick up a test case which covers business logic of the application, e.g. if it is a note taking app, try saving a note.

A test flow which covers the functionality along with dependent component.

There are couple of factors that might affect the selection of our test case :
1. Impact Analysis of current change in a release.
2. Functionality being used maximum times in production by users.
3. Risk analysis.

We should choose the test case which would cover the major functionality used in production since users work would not be blocked owing to it.