Pai is one of my favourite Thai restaurants in Toronto for three reasons: authentic and delicious food, plenty of seating, and relatively quick service. My favourite is the green curry (with shrimp) that comes in a coconut. The curry is incredibly aromatic and even more tasty. I like to scrape out the coconut meat with a spoon and mix that in with the curry sauce, which I actually haven't seen a lot of people do!
The khao soi is pretty good too, as is a beef dish that I forget the name of. I find the pad thai underwhelming. When I ordered it, it was dry and not very tasty, but it could just have been a subpar plate.
You can order any level of spice, from absolutely no heat to Thai spicy (watch out-this one's hot). If you try to come here for dinner on a weekend, note that it'll be VERY busy (expect 40-60min wait depending on group size). However you can leave a cellphone number and have them text you when the table is ready, so you don't have to physically wait there.
TIP: this restaurant has a great private room that you can book for no extra fee (just a set gratuities that automatically comes on the bill). It seats about 20 people max and is nicely decorated.