Probably Arbor Vitae.

Potted evergreens including arborvitae, juniper and others. If you go to your local nursery, ask them to school you in the care of the many examples they'll have already in pots or balled.

Prunus Padus

Thuja tree or Japanese maple.

https://tayloronhistory.com/2015/09/25/balcony-container-gardening-in-toronto/

Would containers freeze on a rooftop in a Toronto winter?

With the possible exception of the arborvitaes, none of the plants suggested are likely to survive a Toronto winter in a container. Containerized plants - and even those in good sized containers - are far more vulnerable to winter cold than their inground planted counterparts. Roots are the most cold sensitive portion of the plant and without the insulating properties of the ground mass surrounding them, they can easily be fatally damaged at much higher temperatures than you would expect. Certainly much higher than the average lows of a Toronto winter.

At the very least, you should be looking at plants considered hardy to zone 4. And even that does not take into consideration the pot and its contents freezing solid.

I'd go a different direction. Castor beans, canna, sunflowers - tall plants that can be grown as annuals.