Agarwal, P. (2005). Ontological considerations in GIScience. International Journal of Geographical Information Science, Vol. 19 (5): pp 501-536
This paper published in 2005 has been one of the most well-cited papers in IJGIS in recent years. It now has more than 170 citations and was recently listed as one of the 35 most cited papers ever in GIScience.
It is also on reading lists for various GIS courses and research groups across the world.