Laowa 12/2.8
Seldom mentioned here, maybe due to avaliability.
More flare tolerant than the Irix 15, smaller but wider.
f2.8 and good IQ (for a UWA anyway)
IMGP996012019-2400 by
pinholecam02, on Flickr
Samyang 14/2.8
A good copy is very good (esp at f11), pretty flare tolerant (better than the Irix)
So so build quality, but with that, lighter weight.
20180208-IMGP3505 by
Pinholecam_01, on Flickr
K28/3.5
Best landscape 28mm (though also the larger of the Pentax 28mm)
Luminous city by
Pinholecam_01, on Flickr
K24/2.8
IMHO, really still ok on the K1
Small, f2.8 + 24mm is handy for landscapes as well as other shooting.
20160905-IMGP3956 by
Pinholecam_01, on Flickr
The advantage with the primes is always that they are modular, so one can always leave a focal length at home when there will be less situations that need it and enjoy the lower encumbrance.
The DFA 15-30 is a handful of primes or course.