I thought of a list of things that would have never happened if Katherine wasn't here, and I came up with a good bunch.

1. Without Katherine, Stefan and Damon would have stayed human and died a long time ago.
Katherine made Stefan die and become awesome, and with that he turned the Damon we all love into himself. So she practically gave us the hot, badass Damon we all adore so much.
2. If it wasn't for Katherine, Elena wouldn't have been even born.
If Katherine didn't have a Baby and that baby didn't have a baby and that baby didn't have a baby etc. there would have been no Elena would there?
3....