I have so many favourites... =(
The only one I have is the 135mm f/2
Intend to get the 70-200mm f/2.8 L IS II (another favourite) soon though...
also:
200mm f/2 IS
300 f/2.8
100 L macro
Anyone want to donate to the "D.Sim Favourite Lens Collection Fund"? 
You can have my Tokina 17mm, lol. It is stuck on infinity, and the AF does not work. Neither is it supersharp. I bought it used, and was screwed over. On the other hand, I paid so little that I did not bother making a fuzz.
While you dream of the 200 F2.0L IS, I would seriously ask you to consider it to try it before you buy it. Its heavy and bulky, and it comes with what my wife referred to as a beauty bag;). It gives incredible pictures, but I believe that the difference between the image quality of that and the 70-200 F2.8L IS II is minimal and does not justify itself as far as price is concerned (unless you do this for a living, or someone else is picking up the bill).
I see many mentions the 35L. I have never tried it, but I was lucky to observe a professional photographer a month ago while he was shooting some pretty awsome portraits/environmental portraits. It is definitly on my list.
I have to say, after reading many of these posts that I am a bit envious of some of you guys who have owned and still own some 'old school lenses', the FD's.