What seedlings do you hate to yank?
For me, it's hellebore seedlings. I love hellebores and think those little seedlings are beautiful. I let them grow when I can, and transplant what I can use, but always end up having to pull some which seems like such a waste.
This year I have a bumper seedling crop and the hellebore seedlings are everywhere, including being intermingled with everything from sedges to hostas. They're deep rooted little buggers, so just a little more effort than some seedlings to get rid of too.
Then there are the maple seedlings...a.k.a. the SE Michigan 2014 plague. I love to get rid of those, but it's not fun. Hundreds. Everywhere. To the compost!