Not particularly sure about the details, but I noticed that there is a train from YYZ to the city last week. I saw the sign when walking to the car rental facility, so that's the extent of my knowledge. I am sure there are others that will be more helpful than I was.
Not sure about uber, taxis are on flat rate and run about $55-60.
I think the shuttle services went out of business when they built the train. The train is apparently nice but on the expensive side at $27.50 one way.
If you really want to cheap out you can take public transportation (TTC) for $3. There's a bus that picks up at the terminal and takes you to the subway. As long as you're not arriving in rush hour it's pretty painless.