The winners were Karen and I/me.

Pretend Karen is not included. Take her out and make 'winners' and 'were' singular.

The winner was I.
The winner was me.

Which one is correct? I think it should be The winner was I.

The two personal pronouns I and me are often used wrongly, usually in sentences in which I is being used with another noun. Here are some tips to help you get it right:

Use the pronoun I, along with other subjective pronouns such as we, he, she, you, and they, when the pronoun is the subject of a verb:

He went to bed.

We waited for the bus.

Clare and I are going for a coffee.

In the last example, the pronoun I, together with the proper noun Clare, forms the subject of the sentence, so you need to use I rather than me.

Use the pronoun me, along with other objective pronouns such as us, him, her, you, and them, when the pronoun is the object of a verb:

Danny thanked them.

The dog followed John and me to the door.

In the last example, the pronoun me, together with the proper noun John, forms the object of the verb follow, so you need to use me rather than I.

Use the pronoun me, along with other objective pronouns such as us, him, her, you, and them, when the pronoun is the object of a preposition:

Rose spent the day with Jake and me.

Me, together with Jake, forms the object of the preposition with, so you need to use the pronoun me rather than the pronoun I.

Myth: Expressions like "It was me" and "She was taller than him" are incorrect; the correct forms are "It was I" and "She was taller than he."

Pullum responds: The forms with nominative pronouns sound ridiculously stuffy today. In present-day English, the copular verb takes accusative pronoun complements and so does "than." My advice would be this: If someone knocks at your door, and you say "Who's there?" and what you hear in response is "It is I," don't let them in. It's no one you want to know.

"People have been living in fear of grammar rules that don't exist," said Pullum, who wrote The Cambridge Grammar with Rodney Huddleston of the University of Queensland, Australia. "We're going into the 21st century carrying grammar books from the 20th century that haven't shaken off grammar myths from the 19th century," said Pullum.