This question is actually pretty tough to answer. Ontario is a very big place and there are so many different things to see and do here.
For most people, Toronto or Ottawa will be the major draw.
Toronto is probably one of the most vibrant and metropolitan cities in North America. If you like the night life, music, theater, professional sports, then you'll like Toronto.
Ottawa, on the other hand, is more suited to those who enjoy history, museums, etc. It isn't as big as Toronto, but as the capital of Canada, it has an appeal all it's own (except for those of us who send our tax dollars there 8=).
Then there are those who like the outdoors. Ontario has plenty to draw you throughout the province. There is lots to do in all 4 seasons.
I live in the Muskoka region, which is about 160km north of Toronto. This is a very rustic area but close to the hustle and bustle of the big city. In the summer the population triples with cottagers and resort guests who come for our beautiful lakes, wonderful provincial and federal parks and some world class golf courses.
The winter season brings us lots of snowmobile enthusiasts and some ski and snowboarders (although there are better facilities for the downhill sports in the Mt. St. Loius/Moonstone region).
Next summer will see the G8 summit here in Huntsville (the northern part of Muskoka).
You can check out the Ontario Government web site to find more information at http://www.ontario.ca/en/visitors/index.htm as a starting place.