I'm not really into the identification of favourites, because how can you manage to compare a contemporary house with, a massive modernist bank tower, with the heavily decorative Confederation Life building, in any meaningful way. So much to choose from, so little basis to make a comparison!
However, I'm going to nominate the R.C. Harris Filtration Plant as my favourite nonetheless, perhaps because I so recently visited the site to take photos of the complex.
1. The scale of the building is both overwhelming (it is massive and takes a good many minutes to walk along the edge of the longest of the three visible structures) and intimate at the same time (some of the decorative details and fountains can only be appreciated up close - and in their scale they acknowledge the size and scale of the persons that might be nearby appreciating them). It is an inviting building, that welcomes your contemplation.
2. It's one of the few large structures in Toronto that truly has room to breathe. Our cultural institutions are generally crammed into sites that, increasingly, see them pouring over the sidewalks, like they are bulging from within (which, happily, they are). I like this, as it is very urban, but it's nice to find, somewhere in the city, a truly grand vista where you can appreciate a building from a distance. Even better, when you are up next to the highest of the Harris buildings, you are rewarded with a view of the turquoise waters of Lake Ontario from a hilltop perch.
3. For me, I can't separate the meaning and purpose of a building from it's aesthetic. The TD Bank Tower, gorgeous as it is, speaks to me of the careful pursuit of private gain - nothing wrong with that, we love it, in fact - but it diminishes the splendour of the building. The RC Harris speaks to me of a communal act of social welfare - the provision of a common good - and that warms me to it. This is a purely person feeling that I don't expect anybody else to share, but I can't separate these things. (For instance, I could never nominate a private dwelling as my favourite building).
4. Finally, I admire greatly, but cannot love, the TD Centre. It's clearly a front runner in any campaign of the best buildings in Toronto (and deservedly so). It sits as the King of the financial district, laughing haughtily at the taller yet lesser towers around it, still the truly commanding presence of our skyline. The RC Harris, existing on the south-west fringes of Scarborough, just past the popular bits of The Beach, is an out-of-the-way, discreet friendly giant, doing it's bit to give Torontonians clean and fresh water - the very stuff of live. It's like a big, slightly stupid uncle that gives the best gift every Christmas.
(PS my favourite contemporary house is 48 Heathdale Road).