it depends first of all (and in fact most of all) on your budget
if u're rich, and have a thousand bucks to spare, get the 17-40/f4L. it's a very sharp lens and provided u don't do too much low-light stuff, u'll love the sharpness, and the wideness of it.
but if u don't have that kind of a budget, then it depends on whether u want faster (wider aperture), or wider (shorter focal length).
u can get the tokina 17/f3.5, which costs about 800 to 900 brand new from AP, the canon 20/2.8 which costs about 800 brand new, or the sigma 20/1.8 which costs about 600 brand new (but it has a 82mm filter diameter so the uv filter will set u back another 50 bucks if u get a non-multicoated one or a lot more if u get a multicoated one).
the sigma 20/1.8 is reputed to be very sharp, n it's very fast. the canon 20/2.8 costs more, n is slower. some say the sigma is sharper, some say the canon is sharper. i don't know
i own the canon 20/2.8, n i find it quite sharp from f8 onwards, acceptable at f5.6, and not quite usable below that (at 100% crop from my d60) but it depends. different ppl have different tolerance levels for (un)sharpness. i use my 20/2.8 for landscapes, so it's either in bright daylight, or i have a tripod, so i'm ok with it. but the canon 20/2.8 has VERY fast AF.
so in the end, it's up to u and ur wallet