It's not goji berry - that has a narrow leaf and quite a slender plant. I'm also doubtful of it being either of the other two, which both have a much rounder and glossier leaf.
It does, however, look vaguely familiar - have you added any home-made compost that might have included store-bought fruit or vegetable scraps?

It looks like a Kiwi seedling.

You know, I planted tomato seeds in a pot and a squash plant is coming up. The reason I think that is happening is because I used some of the compost from my worm bin, and probably had some squash seeds in it. The other seeds I planted (Yellow mangosteen, goji berry, and soursop) were planted in potting soil, but I am not sure if I used any worm compost on it. What I think I did was to use soil that I made into potting soil in my garden, so there probably is some garden soil in those pots which may have contained seed scraps. Whatever this plant is, it seems to be thriving well. But I have no idea what it is. I have had kiwi in the past, and used to put out vegetable and fruit scraps into the garden, but I don't do that anymore, because animals are attracted to the garden when I put any scraps out, and they destroy the plants as they enjoy moving the food scraps around.

It's probably something worthwhile, and I like mysteries, so I'm glad you're going to keep it.