Your favourite piece of glass?

I'm looking for everyone's opinion on their favourite lens, nice and simple really.

I am thinking about getting a few new lenses to go with a new camera, so as we all have our favourite photography styles it would be good to get a broad range of opinions on what everyone uses or has used to take your favourite types of shots, on the flip side perhaps you have had bad experiences with various lenses as well, so some to avoid could also be helpful.

I will be sticking with Nikon for my upgrade and most likely going to go full frames so that's what I'll be looking out for, but if your a Canon, Sony or Pentax user feel free to pitch in, you may help someone else out.

Thanks in advance.