I agree. Execution time is one of the most important. Not on your list is one of my other favorites, frequency of call. Your longest running proc may be called once a year. You may have a very fast proc that is called a hundred times a second. If you can cut either proc's execution time in half, which would be the better target?

But, that said, you have to take it all into account. Just tuning for reads may not solve slow performing queries. Just tuning for speed might not tune queries that are accessing too much data. While I do put execution time & frequency at the top of the list, the entire list has to be taken into account.