Seeking top tier conference in GIScience?
These three conferences are influential in my academic GIScience circles.
(stalwarts) (1) Conference on Spatial Information Theory; http://www.cosit.info/ (2) International Conference on Geographic Information Science (GIScience); http://www.giscience.org/
(rising in importance) (3) ACM SIGSPATIAL International Conference on Advances in Geographic Information Systems; http://www.sigspatial.org/sigspatial-conferences
An accepted paper for COSIT or GIScience might be somewhat equivalent to publication in International Journal of Geographic Information Science or Transactions in GIS (IJGIS Impact Factor in 2012: 1.613 & TGIS Impact Factor in 2012: 0.906).
Of course, the interdisciplinary spatial journals have the highest impact: http://www.aboutgis.com/gis-and-remote-sensing-journal-list-with-impact-factors/
Due to the specialized nature of GIScience, I expect the most-influential papers on geospatial information research will begin to be published in journals with a wider reach. That same gravity also will pull the work to higher impact conferences, such as one of the Association of Computing Machinery specialty meetings -- or perhaps to large conferences, like the American Geophysical Union Fall Meeting.
Simultaneously, the Internet minimizes the need for singular important conferences and journals. In the longer term, I expect closed, expensive, or proprietary channels to be diminished in importance.
My perspective is skewed toward the U.S., but both the AAG and AGU conferences heavily feature the bread and butter of GIS: spatial analysis, remote sensing, spatial statistics, cartographic methods, geographic demography and others. at AAG in particular it is a minority of sessions that do not feature GIS (the toolkit), and there are dozens of sessions on GIS (the science/perspective).
GIScience publishing for maximum cred is fairly venue-agnostic. Conference papers have a fairly equal value with journal pubs when tenure is being considered.
Besides GIScience, I would add SDH (Spatial Data Handling conference), GeoComputation and the AGILE conference, in Europe. More focused on cartography, there are also the ICC (International Cartographic Conference) and AutoCarto conference.
An excellent resource for GIS conferences traking is this conference calendar maintained by colleagues of the Zurich university.