OK, UP. You're not a bad airport option.
Recently, my little fam went on a big holiday and we didn't want to drag along a carseat just for airport-to-home-in-Toronto purposes. I know legally you can take a wee one in a cab without a seat, but it freaks me out on the highway. I didn't want to start les vacances freaking out, so we opted for the UP.
We live in Parkdale so our stop is Bloor W. It was cheap, fast and really clean. If you live in condo land by Union or were coming in on other transit through Union before heading to the airport, this is the perfect option for you.
The only reason it isn't 100% five stars for me, is the Bloor St station is under construction, and it appears even after construction is complete, to get to a main street to get a cab, you have to walk a lot and then through an awkward FreshCo parking lot. This would definitely be a weird welcome for someone for someone thinking they were being spat out in Toronto's trendy west end area.