If I want to use a 10-year ACS file. Which weight should I use?

Some researchers do pool data to increase the sample size. This is essentially what the Census Bureau is doing with the 5-year files, combining five 1% files into a 5% sample. It is important to understand, however, that this is now a sample of the 5-year period instead of a 1 year period. Similarly, if you pool two 5-year files to create a 10% sample, you are now sampling from the 10-year period.

It is important to use weights to make the sample representative of the entire population and account for some sampling error, however which weight(s) you decide to use will depend on your analysis. For some additional information about weights, you can refer to this forum thread.