The best way to implement this requirement is to create a formula field on the Case object using the TEXT function. A formula field can display the value of another field from a related object, such as the Account object. The TEXT function can convert a picklist value into text, which can be displayed on the Case record.
Creating a formula field on the Account object using the ISPICKVAL function is not a valid solution, as it does not show the value of Support Level on the Case record. The ISPICKVAL function is used to check if a picklist field has a certain value, not to display it. Creating a Process Builder and using a field update on the Case object is not an optimal solution, as it requires more configuration and maintenance than a formula field.
A Process Builder may also introduce delays or errors in updating the field value. Creating a roll-up summary field using Support Level on the Account object is not a valid solution, as it does not show the value of Support Level on the Case record. A roll-up summary field is used to aggregate numeric values from child records, not to display picklist values.