Preheat the oven to 400 degrees F (200 degrees C).
In a large bowl, stir together the white sugar and 2 tablespoons flour. Add pears, and toss to coat. Stir in the sour cream and vanilla, lemon and almond extracts until the pears are evenly coated. Pour into the unbaked pie shell.
In a small bowl, mix together 1/4 cup flour and brown sugar. Mix in the butter with your fingers until the mixture is crumbly. Sprinkle evenly over the top of the pie.
Bake for 10 minutes in the preheated oven, then reduce the temperature to 350 degrees F (175 degrees C). Bake for an additional 45 minutes, or until pears are tender.