I'm currently shooting Leica, but I do miss some of the IS Canon super teles. I switched to Leica for many reasons, but bottom line is all the lenses have a very 3D rendering of the world, colors have a lot of depth to them and there is a lot of micro-contrast to the images without them being clinically sharp. These are the most used and pretty much always in the bag:
Leica 15 f3.5
Decent lens, though the f2.8 version is much better. The f3.5 is an older Zeiss lens and works well for landscape use. The f2.8 lens is a more modern Schneider lens and much more expensive. 15 mm is a nice perspective on film, but with the DMR's 1.37 crop factor it is a 21 which is not wide enough for me.
Leica 90 f2 (Summicron, non-APO)
Similar in some ways to the 80 Summilux (f1.4) this lens, like many Leicas, has a bit of a split personality that it is very soft wide open and sharpens up very quickly when stopped down a bit, making it a nice portrait lens. I just like the look of it. I love the way it renders the world at about f2.8-f4 -- smooth but clear.
Leica 180 f2.8
I adore the look of this lens on film, soft with very deep colors. Somehow it doesn't translate that well to digital. Still, it is small, light and fairly inexpensive. Ideally I would love to upgrade to a 180 f2 APO and 180 f3.4, but like the 90 cron above, the 180 does a reasonably job as an all around lens.
Leica 35-70 f4
I add this lens because, even though I have absolutely no attachment to it, there is really nothing to fault. Nothing spectacular, it just gets the job done. Interestingly it is a pretty mediocre lens on film but excellent on digital. I can't really explain it, but my hunch is Leica spent a lot time mapping the lens characteristics to the ROM chip and the DMR puts that information to use well. As a test, a colleague taped over the contacts and fired of a few frames on his DMR and just like film, the lens was nothing special.
Meyer 300/4
This is a 1950's or 1960's Russian medium format lens in a Pentacon mount. The results are, uh, interesting, but kind of fun. I have not completely figured out what to do with the lens since it isn't that sharp and doesn't really handle color that well, but I carry it when weight is not an issue.
Since I am no longer shooting that much, there is some chance that I may sell it all for a small Leica M system and a modern DSLR of some type.