GADM is the most popular among economists. However, for the accuracy of the boundary lines, GAUL appears to be superior: the boundary lines in GADM appear to be drawn rather haphazardly (that is, lots of rather long straight lines) while those in GAUL are very detailed (i.e. lots of vertices).
On the other hand, the attributes of each administrative unit polygons (such as district names) seem to be more accurate in GADM. For example, the parish (fourth level administrative boundary) data for Uganda shows multiple polygons for the same parish name. This is not the case for GADM.
So try different data sources and choose the one that appears to be the best for your research purpose.
the World Vector Shoreline (WVS)
- includes the shoreline data as well.
- used by Alesina, Easterly, and Matuszeski (2006) to measure how artificial a state border is.
- If you're interested in the historical national boundaries since 1945
Global Administrative Areas (GADM)
- an alternative to GAUL. Whether it is better or worse is not clear.
- mentioned by Gleditsch and Weidmann (2012) in their review of spatial data analysis in political science.
- Used by the Gridded Population of the World Version 4 (see here).
- Used by Dreher et al (2015).
- They mention that GADM does not include the second level administrative boundaries (counties/districts) for Egypt, Equatorial Guinea, Lesotho, Libya, and Swaziland.
- Also used by Alesina et al. (2016) to measure inequality across subnational administrative regions (which turns out to be negatively correlated with per capita GDP).
Global Administrative Unit Layers (GAUL)
- Supposedly an annual panel data from 1990, but my own user experience suggests it's really not: district boundaries do not change for the year when it should change.
- Used by Briggs (2015).
The Second Administrative Level Boundaries (SALB) dataset
- compiled by the United Nations
- provides the GIS data on second-tier subnational administrative boundaries (ie. district boundaries).
- I'm not sure whether the GAUL dataset mentioned above incorporates this or the SALB dataset has its original data.
- For subnational boundary changes during early years
- This is the online updated version of the book Administrative Subdivisions of Countries by Gwillim Law (Jefferson, North Carolina: McFarland & Company, 1999).
- provides the list of administrative regions for every country, past and present. Very useful if you need to match different sub-national or micro datasets based on sub-national regions, especially when a country of your interest has changed the boundaries of sub-national regions quite frequently such as Nigeria and Uganda.