Unfortunately i spend way too much time browsing ebay and googling, nothing in canon's lineup can be considered 'exotic' going on other brands...TOZ-500
russian mirror lens, 500mm f/3.5 is damn fast for a mirror lens.
Another russian, Jupiter 13
125mm f/1.5, but it covers a 4x5" film area (35mm FF equivalent framing of 49.8mm f/0.78)
The Hasselblad 1700mmm f/4 (35mm FF equivalent of 1100mm f/2.6)
CRguy's mentioned the Nikkor 6mm 220degree fisheye.
The olympus OM-mount 35mm f/3.5 macro, specifically designed for 5x-12x mag.
The nikkor 80mm f/1.0 (but only at 1x mag).
How about the Leica telephoto system with interchangeable heads and backs
edit: how could I forget the Siglauncher 200-500 f/2.8?
But for canon, i'm saying their 'most exotic' would be the TS-E 17mm, just because it's wide (for people who think 'independent tilt and shift' is exotic, google 'superrotator').
The 8-15 fisheye certainly is unique in that it gets to a circular fisheye (but no, tokina/pentax got in first with 'fisheye zoom').
Canon also had the 50mm f/1.0, but then Leica have the 50mm f/0.95 as a regular production lens.
In terms of 'trailblazing', i'm picking the FD 150-600 f/5.6, first lens in the world with internal zoom/focus, one-knob zoom and focus so the other's on the shutter permanently (and if i ever get a job that pays enough, i'm buying one for a collection).