That is a tough one. My go to lens that is on my camera 90% of the time is my Canon 50mm 1.4. I do most all of my shooting with this lens. It's sharp, fast and the bokeh is dreamy and beautiful.

However, I will say I love to use my 70-200mm 2.8 whenever I get the chance. It is fast, sharp and great for low light and candid shots. It has image stabilization, which rocks, but It's pricey and heavy, weighing more than 3lbs. However, I can't imagine living without it, now that I have had it for so long.