Man tager vad man haver, you take what you've got, as Kajsa Warg said. I wanted something for my Nikon D600 that was wider than my 50mm, but not as wide as my 15mm, fast, but not heavy.
The first requirement narrowed it down to a single contender: my Sigma 24-70mm f/2.8 IF EX DG HSM. The second narrowed it down to nothing.
Then I remembered that the first photos I took with my D600 were done using my Nikon 35mm f/1.8 DX. Heavy vignetting, since the image circle of the 35mm is too small for full-frame sensors, but still - the corners went dark, not black. Excellent image quality in the DX-crop area but horrible outside, but still - if you compose the shots to have the subject in the DX area that'd be fine. After all, you don't shoot a f/1.8 to get tack-sharp depth of field.
So I decided to try it out in anger. The 35mm went onto the D600, and I thought I'd carry it with me a couple of days and see if any good came of it.
I think it did:
Still, it's a poor (or stingy) man's solution. The 35mm DX is about $200, and the 35mm FX (F-mount) is about $500 (and the Nikon Z 35mm S is $700). You can probably get the first two used for a lot less. But if you already have a 35mm DX and have upgraded your camera to FX and don't mind being somewhat restricted in your compositions - go for it.