The best way is a taxi, but it certainly isn't the cheapest.
Airport Expressis a fairly quick shuttle service. I'm not sure it's all that cheap at $20.
If you have time to kill and don't mind transit, there's the 192 Bus from Pearson to Kipling station in the far west end. You can then get on the subway and head east, switching at St. George or Yonge to go south to Union, which is the main train destination. Info here.
Take the TTC Airport Rocket Bus (#192 I believe) which will take you to Kipling Subway Station, where you can transfer to the subway.
At St. George Station, switch subway lines to a Southbound Train heading for Union station (the main train station).
Map is here: http://www3.ttc.ca/Subway/interactivemap.jsp (Kipling is on the far left, you may have to drag it to see.)
Whole trip costs $3 and will take about 45 mins-1 hr at my estimation.
I'm a Vancouverite but I travel to TO frequently. There is of course a balance between "best" and "cheapest." Cheapest is public transport, but it can take a while (depending on the time of day), and it's awkward if you have a lot of luggage. "Best" is a private car/limo, but obviously that's not cheap.
I usually go the compromise route with the Downtown Express - $21.95 one-way or $36.25 return, and it will get you to half a block from Union Station.
Check out this link for all the transport options.
I always figured Orable for a Canadian. Or was it wishful thinking?!