Probably the mint in the roses.

Or the tomato that suffocated a rose bush.

That shamrock-oxalis type plant sold around St. Patrick's Day planted in the shade garden.

I have a yellow and red striped rose with a burgundy, purple, and orange stippled Iris. Blooming together, they are truly hellish.

An 3' tall aloe growing beside a moth ginger. They are hidden behind a shed but aren't plants you would normally put together. They ended up as partners because the neighbors butchered their trees, drastically changing the light and heat conditions. I had to move plants, and at that time, most of the yard was concrete and the choices were limited. The aloe has had to be moved three times in as many years already, and I really don't know where else to put it. The ginger would be a better mover, but there is not enough room for it anywhere the conditions would be good for it to grow.

So, for now, they awkwardly hold hands behind the shed.

Hopi Red Dye Amaranth (deep burgundy) with clear lemon yellow snapdragons. It made my eyes hurt and set my teeth on edge.

The ugliest I ever saw was a row of aloe vera surrounded by orange and purple violas. Beetlejuice would have loved it.

The worst companions in my garden were the crocus I tried to make live in the Bermuda I inherited ( it was my first year gardening ) before tall fescue became widely known. It was adorable in the dichondra however.

A really beautiful rusty orange alstroemeria which I mistakenly planted at the foot of a fuchsia-purple tree rose "Intrigue". As hosenemesis said, it set one's teeth on edge. I could only stand it once, the two blooms together were like fingers scraping down a chalkboard!

I was really sad, because I had absolutely no other place to put it and it was flourishing. Into the compost cart it went, however.

Not mine, but a sight that I pass every day and wonder if they'll pull something out. They haven't yet, so they must like it.....a long row of alternating red and white roses and Bird of Paradise....