First for me is definitely the Fujifilm 18mm. It's the weakest, optically, of all of Fuji's 'XF' prime lenses. Even the kit zoom at 18mm is sharper and less distorted. It has the joint-worst AF along with the other two launch primes. It's not even the smallest and lightest; the 27mm is a proper pancake and manages to be sharper to boot. But an 18mm on an APS-C (1.52x) is my perfect wide field of view for casual shooting and the size is small enough to be easily portable without also feeling too flimsy.
Most Fuji users ditched the 18mm prime a few years ago, but for me it lives on an older Fuji body and that's my go-to point-and-shoot. I could put on any other lens and get sharper pictures, less fringing, less distortion, faster and smoother AF. You name it. But the 18mm lives on the old body because it feels right.
Second, Canon's 100mm f/2. It doesn't give you the compression of the 135mm or any of the 70-200s. It's more limited in framing than any of the 85mms. (Or, again, a 70-200.) It's an older design with a bit too much plastic. But, like the Fuji 18mm, the Canon 100mm f/2 just fits my eye perfectly. Stick it on a 35mm 1D body and the image in the viewfinder is exactly what my eye sees, to the point where I can keep both eyes open and I don't experience any double-vision at all. The AF is that hair faster than the 100mm f/2.8L Macro. It's sharp where it counts. It has no distortion or aberration on a 35mm body and only minor fringing on an APS-C body.
Every time I pick it up, I know there's a better lens to use. I could pick up the 135 and get better compression and a sharper image. I could pick up the 85mm and capture a wider variety of shots without needing to change lenses again. I could use a 70-200 and blow everything out of the water. I could use the 100mm L macro and have less flaring.
But that basic, cheap, 100mm f/2 is just too lovely. It's pointless, but lovely.
Third, the Mitakon 35mm and 50mm f/0.95s. Totally pointless lenses. Optically a bit weak. Awkward aperture ring placement. Heavy, dense lenses for their size. There are better 50mms (the 35mm is for Fuji, where it's a 53mm equivalent) for every system. The f/0.95 isn't even really that fast because it's actually t/1.3-1.4. But the focus rings are beautiful and using them makes me feel like I'm back using my dad's Canon A-1 and 50mm. (Which I did dust off and retry a few years ago and found that is nowhere near as nice as I remember it being; the Mitakons are as nice as I remember the A-1/FD being.)
Totally, totally useless lens. A Canon 50mm f/1.8 STM or Fuji 35mm f/1.4 would beat them if you want objective quality. If I really need a great 50mm for a job, I use the Sigma 50mm Art, or for a couple of specialist roles, a modified Samyang 50mm. I've had the 50mm f/1.2L and f/1.4 a few times each and those are (kind of, bar a few flaws) better lenses than the Mitakons, too. But the Mitakons are the ones I like using most so they stay in the bag.
Fourth and last, Mamiya 110mm f/2 for the 6x7 system. In fact that whole system is useless since a 6x7 digital sensor doesn't exist and 6x7 film is unwanted. I've adapted it with a 6x4.5 digital back but that too still leaves it useless as I have a much more capable Phase system anyway. But, I keep the Mamiya maintained in good condition just so I have a way to use the 110mm lens, which equates to roughly 60mm f/1.1 in 35mm sensor terms with the 6x4.5 back. (It's 55mm f/1 with the original 6x7 format.) Optically the best lens I've used since I first learnt the basics of photography with my father's large format rig ~25 years ago. No distortion, no fringing, and the best resolving power edge-to-edge of anything I've ever been able to test under reasonably comparable conditions. I get more detail out of the Mamiya 110mm and a 50mp Phase back (at reasonable ISOs, of course) than out of the Sigma 50mm Art and 5DS R.
The whole system is redundant, slow and out dated, a pain to set up and use, and irrelevant. But that one lens keeps it alive for me and if there's a fire here and I have time to pick up one camera and one camera only, it's going to be the Mamiya with the 110mm attached.