I'm pretty sure there are a couple of different cuts that are called "Chuck roast" But they will be labeled as a Chuck roast. I can't really offer which particular cut is better. I would look for one that has decent marbeling.
Usually the under blade roast is labeled as chuck roast. This is what you want. In our stores you typically see
1. Chuck roast- which is the blade or under/bottom blade roast. This is what I use most often
2. 7 bone chuck roasts- which includes the blade which is shaped like the number 7
3. Shoulder roasts- leaner than the basic chuck
4. Top blade shoulder roast-which is where the flat iron steaks are cut.