Even though there have been already a couple of answers here my thoughts (being now 10 years in the research area). However, just before, I might say that locally there are important conferences - with fewer international participants, due to availability of travel budgets. Also it is probably important in what GIScience application field you work. E.g. if you are in cartography, ecology or transportation, etc.
So from my experience, and where all the top researchers go is definitively
(1) GIScience - which is every 2 years.
(2) In every other year there is Conference on Spatial Information Theory - COSIT. So, lots of top people come there too.
(3) ACM SIGSPATIAL (every year), with very technical audience (new algorithms - people from Oracle, Microsoft, Google, etc. may be there)
=> Placing some presentation in either of those conference is an achievement and can be counted like having a journal article. However, if you do not make it into GIScience, etc. with your submission: don't worry too much. I think there is always double the number of submissions than spots for presentations. Then, you could still look what workshops are there at the same time and perhaps present there.
Then there come other conferences that have a long tradition and which I not may rank as either one being better:
(4) Conference on Spatial Data Handling (SDH, every 2 years), AGILE (every year), GeoComputation (every 2 years) and AutoCarto (every two years?).
(5) If publication is not so super important, but you want to see and learn something and make some contacts (I enjoyed it a lot as PhD student), then: go to GIS Research UK (GISRUK, every year). If you are interested in open source GIS then FOSS4G (every year) should also your choice to meet people with similar interest, or the other person that you talked to only via email for years.
Newer conferences that could be of interest are: OpenSource GIS (OSGIS) UK and Open Source Geospatial Research and Education Symposium (OGRS). Participation is most likely worthy if you live around the area where it happens.
Well, and if you are in Cartography, Mapping and GIS, then the Int. Cartographic Conference (ICC, every 4 years) should be on your plate as well. Similarly holds for Remote Sensing and GIS related work with the ISPRS Conference, which is also every 4 years. For Surveyors this may be FIG (though, I don't think publication is very important here) and for everyone in spatial data infrastructures this is the GSDI conference series.
For people in North America the AAG (Association of American Geographers) Conference (twice a year?) is of interest. And if you are more into Remote Sensing then it is the ASPRS Conference (every year) that should attract you. Likewise for folks in Germany, Austria and Switzerland the AGIT could be a place to go and meet up. Canada has actually SKI (Spatial Knowledge and Information).
I am sure I forgot some conference, but... well, this is why others can write here as well. At the end you can visit each year lots of conference and workshops. It all depends on your travel budget. Also, you may have a look at the workshops that are run together with the bigger conferences (i.e. some form of specialist meetings and an even better place to meet people from your subfield). They are a good indicator about how important a conference is - just check how many workshops are associate with the GIScience Conference and who organizes them.
What is finally important: I was close once to submitting something to a conference or workshop that I was in doubt about the credibility (that may you just make you pay lots of cash - but are in locations your always wanted to go to: like Brazil or Dubai or so..). So in that case I looked up on who is on the board of the conference committee (for reviews of submission, etc) - but not part of the organizers - and if I knew a name of the list from some publication. Then I wrote to a person I had heard off or knew and asks if he/she would go. Then, depending on that persons response I would submit or not.
So, and if someone reminds me, I would also write in a bit about my impression of the journals to publish in. However, as someone mentioned above, I feel the most impact can be achieved if you publish in a journal of the area that your GIS work is related to: that is (a) check where your readership might be, and (b) how technical is your article.