I need the historical GDP of all modern countries (or at least the vast majority) from 1945 to now. It seems the world bank data only starts in 1960 or later. I found Angus Maddison's data, but I don't know how reliable it is. Can anyone tell me if it is reliable? Are there other good data sets for my needs?
As far as I know Angus Madison's GDP figures are one of the most reliable sources of historical data, even if data construction is as always diputable. The Review of Income and Wealth published a rigorous debate between Angus Maddisson and Carsten A. Holz regarding Maddison's estimates on China's growth.
You may also find useful the Historical Bilateral Trade and Gravity Data set (TRADHIST) provided by the CEPII, which partly uses Maddison's data. TRADHIST gathers more than 1.9 million bilateral trade observations for the 188 years from 1827 to 2014. TRADHIST also provides about 42,000 observations on aggregate trade, and about 14,000 observations on GDPs and exchange rates respectively. This data set is publicly available.
Another source is the International Historical Statistics 1750-2010, a massive handbook of all sort of statistics, including GDP, for virtually every country. The data is full of references and footnotes to indicate the source and any adjustment made to the data, so everything is crystal clear.
Two drawbacks are:
This data are not publicly available, but if you belong to an educational institution it's likely you will have access to it through your local library.
Data comes in pdf. You can however easily copy from pdf to excel using tools like this one.
Regarding to criticism of Agnus Maddison data, according to this source (sloppy but at least provides references) the major issue is related to old data. You would expect modern data is taken from accurate sources. In any case, documentation is not very easy to navigate (e.g. see here). In that sense, IHS is way better in my opinion.