Depends a little on how you shoot. I prefer longer lenses - especially the 85mm and 90mm lenses. 135mm is perhaps not ideal on a RF I think as its a bit too long but if you don't mind the extra length the Canon 135mms are superb - both the chrome and black/chrome ones although the former are heavy. I have had both and still have the black and chrome one but do not use it as much as it deserves. It is sharp!
The 85mm lenses made by Canon are a bit expensive but nice. I owned a 85mm and used it on a Leica M with adapter but found that it was back focusing rather badly so ended up selling it (which I still regret. I imagine it would be OK on a Canon LTM camera).
My ideal 50mm for the Canon is the black chrome 50mm f1.4 which I prefer to the 1.8 - somehow it has a nicer character, I find, even though the 1.8 is a superbly sharp lens. I cant put my finger on it but there you go! Perhaps its just the extra separation that comes from having a wider maximum aperture and the softer bokeh. If I owned both and had to sell one I would definitely sell the 1.8 not the 1.4. Actually the acknowledged best Canon 50mm is the chrome f.15 in Sonnar design. Problem is everyone now knows how good it is and the price has skyrocketed putting them out of the range of many people and making them hard to find.
And if you want a lens that is not too expensive, easy to find and which shoots nicely when shot wide open the 50mm f2 Summitar. A slight problem is that the filters can be slightly harder to find than ones for the Summicron but you can also find on eBay, an adapter that allows the use of the latter filters if that's your desire. It may scrub up as a better option than your current Canon 50mm as its bokeh is more distinctive. If you do go down this path, I advise buying an early post war coated version. Prewar ones are un-coated (unless they went back to the factory after WW2) so you may wish to avoid those unless you like low contrast images. Also the later version of post war Summitars had a kooky hexagonal aperture blade design that would not produce the soft bokeh of the earlier versions (in which the aperture is perfectly round). In discussing which serial numbers had which aperture blades a post on one forum says that "somewhere between 792XXX and 7978XX (ca.1950) is the turning point between round and hex irises"
You are fortune that there are any number of good lenses able to be used on an LTM camera.
