My nominations for plants that just can't be beat:
1. Nepeta. Wow. If you have space, this is a great plant - nothing bothers it, it grows in clay, lots of sun, lots of water, no water.... mine have withstood just about any abuse but foot traffic. I haven't tried it in a part-sun situation - any experiences?
2. Pulmonaria. This was a complete surprise to me. I planted Trevi Fountain, Mrs. Moon and Bubblegum this year - in a VERY hot July, thinking they were goners, especially with my brand of benign neglect. They have not only survived, but thrived and grown so well that I'm going to have to move them to a spot where they can spread better - they're a little cramped!
3. Heuchera. You all probably know this one already - I love, love, love this plant. It loves my awful clay soil, takes periods of drought in stride and the changing colors keep me watching them. They're nearly evergreen here. Favorite: Georgia Peach.
Anything in your garden that can take some abuse?