There's a lot of competition in the court at First Markham Place where many vendors offer cheap bubbletea with tons of flavours.
I don't have a favourite spot but I can say that Leaf DeTea's bubbletea comes out consistant each time I get it and the milk tea special for $2 including tax is amazing...what I get when I'm in the area.
They also have a deal with curry balls and a large bubbletea for $5; a great snack combo!