Any that you spent too much money on, or that you really, really want. In all seriousness though, like others have said, it just depends on the tank. There are a few corals that have come from very reputable sources that no matter how many times I try I cannot get to keep colored/survive in my tank. Other corals that a buddy has a hard time keeping grows like a weed in my tank. I also have not had good luck overall with wild/mari stuff, but have a few that have done really well for me.
If you want to try SPS, find someone local that will give you a few cheap frags to try and see how they do over the next couple of months. Use that time to learn more about these rewarding, yet frustrating consumers of calcium that take up real estate in our tanks.