I like my EF 20mm f2.8...
Pacific sunset
EF 20mm f2.8 at f11. EOS 5D Mark II at ISO 6400, 1/30 shutter speed. Handheld, avail. light. Pigeon Point lighthouse
EF 20mm f2.8 lens at f11, 72mm B+W Kaesemann C-Pol filter. EOS 5D Mark II at ISO 200, 1/60 shutter speed. Handheld, avail light. It's a fine lens on crop cameras, too... just not very wide.
There are plenty of options, primes and zooms.
Sigma 12-24 (the widest FF lens made, aside from a fisheye)
EF 14/2.8
Rokinon/Bower 14/2.8 (manual focus & aperture)
Zeiss 15mm (supposedly coming soon, manual focus)
Tokina 16-24/2.8
EF 16-35/2.8 II
EF 17/4 TS-E (tilt-shift, manual focus)
EF 17-40/4
Zeiss 18/3.5 (manual focus)
EF 20/2.8
Sigma 20/1.8
Zeiss 21/2.8 (manual focus)
If you consider used and adapted vintage lenses, there are more.
It's hard to call one or another "best".... Depends upon what's important to you. Prices range from a few hundred dollars to more than $2000. The wider the lens, the more distortion... usually. The closest to your 17-55/2.8 is the 16-35/2.8 II... in terms of focal length, max aperture. On your present crop cameras,few of these... not even your 17-55... are particularly wide... most are only moderately wide at best. Super or ultrawide on crop cameras starts around 15mm and shorter. There's even an 8-16mm now!