Best WA Prime or WA Zoom for high Image quality on 7D?

Im looking for the best solution for a high quality UWA or WA lens be it a prime or a zoom for my 7D ..

image quality is important! sharp into the corners would be very nice, i have had a Sigma EX 10-20 f3.5 it failed somewhat in the corners with slight but noticable blur but otherwise a very fine lens.

i have come to the conclusion the Canon Ef14 f2.8L is probably it but at a huge expense? with an aprox 22mm FF fov.

surely there are other alternatives be it UWA zooms or primes im looking for a 20mm to 24mm fov FF equivelent, lens is for Landscape photos mostly.

i would like to keep the price down to below £600 if at all possible?

the Canon Efs 10-22 has been advised but its now an older lens and probably behind in IQ compared to more recent lens designs (i assume?

thank you!
Michael
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Eos 1v Film SLR now covered in dust, sad but true.....