My worst was a chain hotel in a popular area. Unfortunately it was disgusting. Paint flecks falling off the ceiling, carpet that turned my baby sister's feet black from how filthy it was, dust and dead bugs on the (inside!) window sills. But then at night the live bugs came out. Carpenter ants were everywhere. We all had bites. Thankfully we were able to argue our way out of that place for the duration of our stay and into a nice room in a different hotel that was owned by the same business chain. It was ridiculous, not only how disgusting and unsuitable for guests the first place was, but how unapologetic and unhelpful the staff and management were when we complained.
So on the flip side, my best experience is a simple hotel that has really great maintenance and hospitality. It's part of a chain, but feels very personal. It's reasonably priced, kept clean, and cozy. They have a fireplace in the lobby and a free library shelf. They put out fresh fruit in the day and baked cookies in the evening for the guests to take. You can always help yourself to coffee or tea. I once came down to the lobby to read my book in an armchair late at night and the woman at the desk offered to fetch me a glass of milk to go with my cookie! It was just all very sweet and I'm loyal to that hotel now when I travel to that city for business.