I’m looking for an annual for Toronto, partial shade and moist heavy soil, with ample, strongly coloured blooms in a low, mound shape. The planting area is at eye level. We tried single begonias this year, but a lower, round mound would be better.
A good choice for you might be the New Guinea impatiens, Impatiens hawkeri, a very reliable bloomer with a compact, round shape. It grows well in containers. Another lovely annual for shade is browallia, which also has a compact, mounded form and flowers profusely. Most flowering annuals prefer well-drained, fertile soil, so you should consider amending your heavy soil with organic material to give your plantings their best chance to thrive.
Some other ideas for your partial shade location can be found here, in the Toronto Master Gardeners Guide to Annuals for Shade.
Here is a link to more information on Impatiens hawkeri: http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=b665 and here is a detailed description of Browallia: http://www.gardening.cornell.edu/homegardening/scene3b12.html