My favorite lens of all time is the Sigma 85mm f/1.4 Art series lens. The only limit to this lens is the person using it and I certainly have my limits but it is a phenomenal piece of glass that has really opened up my eyes as to what can be captured. If I could only use one lens for studio photography it would definitely be this one. Check out the specs on the Sigma website and read this awesome review on Digital Photography Review. Click the image to be redirected to Amazon for more details.
Don’t forget to protect your lens with a filter. I use the 86mm German Optics because it has great reviews and is at a really good price point.