GADM is the most popular among economists. My experiences of using both GADM and GAUL show that GADM is indeed more trustworthy than GAUL.
- GAUL's parish (fourth level administrative boundary) data for Uganda shows multiple polygons for the same parish name. This is not the case for GADM.
- GAUL's coastline on the south of Baku in Azerbaijan is drawn where elevation (according to SRTM30: see this post) is above zero meter while GADM's coastline is drawn where elevation changes from positive to negative values.
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 the district boundary changes are properly tracked only for some countries.
- 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.