I'm curious – what great 3rd party lenses are there for which there are no Canon equivalents? The optically stellar manual focus Zeiss lenses come to mind (although there are Canon L-series equivalents, just slightly less optically impressive and much cheaper). There are a handful of 3rd party options for lenses like the TS-E lenses (Schneider) and MP-E 65 (Venus, Mitakon), but again, Canon has versions.
The converse seems more true, for me at least – there's no 3rd party 600/4 for Canon, no 3rd party 11-24 or fisheye zoom, no TS-E Macro lenses.
Well Sigma 14mm 1.8 and sigma 20mm 1.4 both helped me make some nice shots in low light situations. And I don't really know what the Canon equivalent is for these lenses.
I also used Sigma 20-35 f2.0 for a couple of low light reportages.
I don't own a 120-300mm 2.8 but I can imagine a couple of scenarios where that lens would be usefull.
And then there are a bunch of lenses that have a Canon equivalents or something in similar focal range, but are cheaper and do the job just as well. Sigma 40mm 1.4 is one of those lenses that I use a lot. It is really sharp and I actually sold my 35mm and 50mm to use the 40mm insteead.
And then the Sigma... 135mm 1.8
Sure Canon has an old 135mm 2.0 model, but it is a bit outdated and too soft for my taste.
Since mirorless cameras have a different AF system I think that makes fast aperture lenses (where focus accuracy is critical) much easier to use. And therefore maybe gives companises even more incentive to produce some niche fast aperture third party lenses.
Heck, I even find I use my 24mm TS-E more just because now I also habe the option to zoom in preview the focus in the viewfinder while shooting handheld.