So what have you killed so far this year?
Me? A fernleaf peony. Assuming it's totally my fault. I've heard it's a finicky plant, but I moved it around a couple times in the garden over the course of two seasons anyway, because I didn't like how it looked in various spots. I finally potted it up last summer because I just couldn't figure out what to do with it. I grew nicely in the pot until going dormant (per usual with this plant), and I stored it over winter same way I store all my other potted plants - all of which are up and running this year except the fernleaf. If it's not up by now, it's not coming up. RIP.
I have a couple of asters that didn't make it, but I don't blame that on myself - they seem to be tricky (? hardiness vs. fall planting issues). But I really love that shot of color in the fall, so I just replace every fall - I just tend to treat as an annual and am pleased if one does actually make it over winter (actually, one did this year!).