Every Pentax shooter owes it to themselves to try at least one Limited lens. They are all special. But I don't know that you need to drive yourself crazy collecting them all.
My three favorite lenses:
1. SMC DA 10–17mm fish-eye zoom
2. SMC DA 21mm Limited
3. SMC DA 15mm Limited
I also have the HD 15mm Limited, but I never use it. It's not nearly as cool as my green 15mm Limited. Watch this video from forum member SnappyPic, which happens to feature my 15mm lens.
If you have and like the fish-eye and the 21mm, you can probably get by without the 15mm. The 15mm is very nice however. While the fish-eye is wide it is, well, a fish-eye. So I'm not sure I'd call it a substitute for the 15mm Limited.
---------- Post added 01-12-2021 at 03:07 PM ----------
FWIW the starbursts on the SMC DA 21mm Limited are just as nice as those on the 15mm. They have the same (or very, very similar) aperture designs.