Love triangles. The ones that worked for you, and the ones that didn’t.
One of my favorite love triangles is the Alyssa, Jeb, Morpheus one, from the Splintered series by A.G. Howard. Jeb and Morpheus are polar opposites, but they are both completely competitive.
Also the triangle in the Soul Screamers series by Rachel Vincent. I absolutely love Nash, Kaylee, and Tod. The way these three interact is priceless.
I can’t think of any that I just absolutely hated.