From Pearson Airport (YYZ):

The fastest and cheapest way into the city is the UP Express, located outside Terminal 1 in Pearson airport (follow signs for "Train to City"). Trains leave every 15 minutes and tickets can be purchased at machines on the platform ($9).

From Billy Bishop Airport (XXX):

Billy Bishop offers a free bus shuttle to Union Station, if desired. You're also pretty close to uber blah blahblah

From Union Station:

XXXXX

From Within the City:

Uber!