My favourite lens in my bag right now is my Canon 50mm f1.8 II. Dollar for dollar this lens just simply can't be beaten. It is super sharp and resolves some silky smooth bokeh. I got this 2 weeks after getting my camera and I am still amazed at the low light performance on my 600D. Where my bigger, more expensive lenses falter this thing just keeps swinging away.

Having said that I am yet to have the fortune of using a fast zoom like the 70-200s or 24-70s so I can imagine my favourite will shift once I have the budget to add one of these to my bag.