I would not take Highway 99 through Whistler. The drivers are insane, the road is going to be a constant construction site until just before the Olympics in 2010 and the climb out of Pemberton is quite steep.
That leaves three routes out of southern BC. All have their good and bad points. Hwy 1 through the Fraser Canyon is probably the prettiest and has the most gradual elevation gain. The biggest problems are occassional narrow shoulders and the seven tunnels between Yale and Boston Bar. They are quite narrow and I would not like to be in them with any transport trucks. However only two of the tunnels are long enough to be a significant problem. And the longest tunnel allows you to look back about a kilometre to see if there is any traffic coming. Look for a break in traffic and cycle like mad. Many cyclists go through these tunnels each year and in the 28 years that I have lived in the area I have not heard of any cyclists getting hurt. And contrary to what Jamawani seems to imply there are plenty of views of the Fraser and Thompson rivers if you travel on this route towards Alberta.
Hwy 3 is also quite pretty but it is also narrow in parts and is the steepest of the three routes. You have nice views all along this route. If you are going all the way through BC using this route the view will be quite nice but there are a lot of passes to climb.
Hwy 5, the Coquihalla, is also quite steep climbing out of Hope but once you reach the toll booth the ride gets much easier and there are ways to bypass the steep climbs before and after Merritt. The nicest part of this route is that there is plenty of cycling room at the side of the road. When I cycled to Calgary with my kids several years ago we chose this route mainly because the highway is relatively new and they built it with cyclists in mind. There are some very nice views along this route but not as many as along Hwy 1 or 3. This is also the busiest of the three routes.
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