Well for shear visual effect, the 200/2L.
200mm is an excellent focal length for compression, and at f/2 the background blur is well into 85/1.2L territory.
However, it's large, expensive and weighty.
The 135/2L on the other hand is light, super sharp, focuses very quickly and being black, somewhat less imposing, it's also a LOT cheaper.
I guess, if I had the money, I'd personally go the 200/2, but if I was looking for a normal portrait lens, the 135/2 is hard to beat in that focal length.