In praise of primes - which next?

I have been a Pentax SLR shooter for over 25 years and throughout most of that time have used zoom lenses.  My main walk around/travel lens is still the 18-250mm and until now I have been content to forgo some quality in return for the convenience this offers.  Recently though I have felt a yearning for better IQ and bought a DA 21mm Limited - the difference in image quality is much better than I had expected and I thoroughly enjoyed working with a fixed focal length.  I am now looking to expand my range of primes - but which to get next?

I mainly shoot travel / landscapes/ urban scenes - some examples with the 21mm from a recent trip to London, below

I have decided that I want to get:

• Either DA 40mm Limited or DA 50mm (yes, I know it's got a plastic mount but I'm comfortable with that - IQ has been excellent in reviews.  I'm equally not concerned about the difference in max aperture, nor do I want the 40mmXS )
• DA 70mm Limited
• [I might add a DA15mm Limited further down the line]

In terms only of focal length, which combination is the most versatile - 21/40/70mm or 21/50/70mm?

Those of you who use primes, which lens do you find the most useful 40mm /50mm or 70mm (i.e. which one to buy first)?



National Gallery



Trafalgar Square



Fountain - Trafalgar Square

--
Confused of Malvern
'The greatest fool can ask more than the wisest man can answer'