If I could only choose one EP, it'd be Ambitions. It was bundled with the base game I bought when I initially started getting back into the series, and having since bought the other EPs through Generations, it's the one that I don't think I could live without at this point. Sculpting is probably my favorite skill, I love the fact that you can be officially self-employed, and I really like the professions a lot. The free clinics that doctors have (usually at the graveyard?!?) are entertaining too.
However, Generations is great if you're really heavy on the family aspect of it and yeah, that spiral staircase is pretty awesome. I like World Adventures too, especially the Martial Arts skill, but it's almost a different style of gameplay if you're doing the adventures - there are some nifty furniture and building pieces, but I don't know that it's worth getting just for that in my opinion. Then again, I'm not much of a builder so I'm probably not the best judge.