As someone said, you'll likely get a number of suggestions based on what people have themselves. But what is the best? Do you always have to go for the best? Do you have the wallet for that?
I have my 35L which I am very happy with, it makes me happy every time I put it on the camera. Always brings home very nice artistic pictures. Maybe not perfectly sharp every time, but I am not always after that. I think you should get yourself a Sigma though. It seems to be the one everybody talks about as the best, so it should suit.
It feels good that I could mention my favourite lens that I've taken my best pictures with in my last post here.
Just remember that it's not always about getting the sharpest picture.
All the best to you guys and take care!