Among other awesome glass, I've got a Zeiss 35mm f/1.4 Distagon.
It's the old Contax/Yashica mount version. It says "Made in West Germany" on it & is built like the proverbial brick latrine. Since it's an "obsolete film camera lens" I picked it up for chump change. I've got about 1/3 as much invested in the EOS adapter on it as I put towards the lens.
I keep it at hyperfocal at f/4 or f/5.6 most of the time, except in low-light scenarios. I admit that other than the first year or so after I got it it mostly lived in the "storage" camera bag as it's pretty heavy compared to most of my Canon primes.
Then I got a 6D, and started using it to shoot short video clips. The "look" this thing gives combined with the short DoF & really long focus throw makes it completely awesome for video. The old-school straight aperture blades make for starry highlights too, which may be spiffy or a hinderance depending on the situation & personal preference.
May I suggest that you want to part with these things because you feel you have a ton of cash invested in them without a corresponding return? Personally my mantra with lenses is "if you've got a good lens & like it, then keep it unless you get something else that totally obsoletes it. & if you've got an emotional attachment to it (for the images you've made with it, not because it's a nice object), then keep it anyway as you'll regret selling it" (my profile icon is a long-lost friend the T90). If you can afford to do that of course; if you're out of work & the rent is due then you've got some tough choices to make, so start with the ones you use the least.
I can't speak for the 15mm or 135, but the c/y mount 35mm f/1.4 Distagon really inspires me when I use it, especially for video. If you get similar pleasure out of your 35 then keep it.
Personally my experience after having owned a bunch of manual focus glass is that the wide-angles are easy to use & get used a lot, and the telephotos are frustrating for anything except controlled situations like portrature where you have the time to review & go back & do it over again & again until you get what you want. Speaking as a long-time owner of the Canon 135L, if the AF is adjusted to where it really hits right I get as much joy & satisfaction out of using the Canon 135L as the Zeiss 35. Those are probably the two stand-out lenses in my bag that I really try to use as much as I can (whenever the situation calls for it). I also really love the Canon 85mm f/1.8 USM as it's sort of the little brother of the 135L, and the 40mm pancake "lives" on the camera as it's a really appropriate length for a lot of situations & it makes the 6D small & light enough that I can comfortably carry it in a coat pocket or messenger bag (much to the chagrin of my Fuji X10 which has seen basically no use since I got the 6D/40mm). The 20mm Voigtländer also fits into this category.
The 85mm USM and 20 & 40mm pancakes all fit into one Canon lens bag which I can take pretty much anywhere if I need to. In fact ever since my 24-105L developed an electrical short that prevents it from zooming beyond 24mm, that's 20/40/85 combo in the lens bag is pretty much my go-to travel kit. The 135L and 1.4x TC go into the shoulder bag as well when I think I'll need anything longer than the 85.
Anyway, with that little tangent over, IMHO: sell the ones that inspire you the least, and do check out the Canon 135L as it's more versatile than the Zeiss & you might even make money on the replacement. With a little luck Canon may even release an IS version now that there's some healthy competition in the "fast 135" space...