Lens for landscape and astro photography.

As a new owner of the 5D mk2 I am pretty dissappointed with wide-angle lenses for Canon FF.
Apart from the high price, the quality is average.

For the astro - big aperture is needed (and quality at that aperture), for the landscape - ability to use filters would be very nice.

So, for the moment I looked at the following lenses:

Canon 17-40 f/4 - too dark for astro; strong vignetting and extremely poor sharpness in the corners at f/4.

Sigma 12-24 f/4.5-5.6 - way too dark for astro and poor sharpness wide open.

Canon 16-35 f/2.8 - extreme vignetting and bad sharpness wide open.

Tokina 16-28 f/2.8 - generally better than Canon 16-35 but can not take filters.

Canon 14 f/2.8 - good, but not a zoom, can not take filters and costs 2000 euro.

Zeiss 21 f/2.8 - good, but not a zoom and seems to be not wide enough.

Canon 24 f/1.4 - good starting from f/2.8, not a zoom.

The questions.
Which one would you choose?

I incline to Tokina and/or Zeiss, or Zeiss (for the night sky) + Sigma (for the day)

Is 21mm enough for night sky, milky way? (I only had appr. 27mm before and it was not)
Is there any way to put polar/gradient filter on Tokina 16-28?
May be some other combinations?
Other advices?
Thanks.