James, don't hold back that way. I think that you should learn to cast aside this staid and reserved form of self-expression and tell us how you really feel.
Plants do evoke incredibly strong positive and negative reactions for me too. Most of the ones I really despise are things that I've had to work very hard to eradicate at some point like a Fuschia magellanica that took over one small shady garden and enveloped it in a dense gloom.
My greatest disdain, however, is reserved for Lysichiton americanus. The common name, skunk cabbage, says it all. This is beneath all weeds in my esteem. It smells terrible and when I was a kid on the west coast of Canada, about the worst thing you could do to another kid was to make him eat some. I don't think that you can imagine my shock and incredulity at finding it as a cultivated plant in this country.