Endpoint URL values to log in to org using web-service API

Salesforce does not recommend "hard-coding" the endpoint URL value into your integration code. The best practice would be to execute the login() call.

This call would return a LoginResult object which has a metadataServerUrl string type that holds the relevant endpoint. You can use this value to reset the SOAP endpoint to the returned server URL.

Having said that, the following endpoints would also initiate a successful login.

For production instances, https://login.salesforce.com/services/Soap/c/xx.0/0DF300000000xxS
For sandbox instances, https://test.salesforce.com/services/Soap/c/xx.0/0DF300000000xxS 
Or, if MyDomain is enabled on the org, you can use,  https://MyDomain.my.salesforce.com/services/Soap/c/xx.0/0DF300000000xxS 

Note: Replace "MyDomain" with the My Domain name of your company.