The correct article to complete the sentence is: the

The sentence would read: "This is the worst performance I have ever seen."

Here's why:

• "a" and "an" are indefinite articles, used for unspecified or first-mentioned singular nouns. Since the speaker specifies "the worst performance," it implies a specific performance they are comparing to previous experiences.
• "some" is even less specific than the indefinite articles and would not convey the intention of pinpointing a particular performance as the worst.
• "the" is the definite article, used for specific or unique nouns. In this case, the speaker highlights the performance as the absolute worst they have ever witnessed, making "the" the most appropriate choice to emphasize its distinctness and superlative nature.