I sold my 16-35 f4L IS USM (the best Canon uwa zoom, next to the 16-35 f2.8 III) cause I love speciality primes, eventually I found the Canon too expensive for only 1 lens.
Only since the 16-35 F4L IS & F2.8 III the corners are sharp on the more expensive zooms.
For the same price I bought 2 hidden discontinued gems, I will compare them side by side.
-The Tokina accepts filters, is the lightest, very compact, great IQ and cheap prime ($250 secondhand)
-The Sigma is very rare, it accepts no filters (bulb type), 14mm extreme ultrawide, fast AF, great IQ and has some flare but not problematic ($480 secondhand), from f8 corners come close to the Nikon 14-24 (see picture below on the right).
I read many user reviews and tests, both have the best reviews qualitywise (I saw there also existed an old Tokina 14 2.8 with AF, but this one was from the pre-digital age, the Sigma is newer and better)
I use my UWA lens for abandoned houses, castles, and other buildings... mostly at f8-f11 on tripod in a bit dark conditions, so I won't use them wide open.
But also for modern architecture during the day it's great.
14mm vs 17mm can make a big difference in big spaces inside
Tokina 17mm review : https://www.slrlounge.com …enses-the-complete-guide/
Extreme or ultra-wides are very hard to find (affordable with AF & least distortion).
With this I want to show that it's not impossible to find an affordable 14mm-17mm prime with AF and sharp corners today if you check the discontinued secondhand gamma.
My friend uses the Samyang/Rokinon 14 2.8 but sometimes misses the AF.
For portraits I have different portrait primes too.
I love ultrawide photography, so possibly I'll keep them both, i'll see my personal testings
Below you see the comparison of the image borders of the Sigma 14 2.8 (100% crops with a Canon 5D at f/2.8 vs f/8)
The center is very sharp like most lenses, but the corners are important too.