+1 for the Tokina 11-16. Good stars at 11mm and wide open. Mine is a Mk1 on a 40D. A quick google says the Mk2 has a better motor and some coating improvements.
Jerry Lodriguss has tested a few lenses and the results for the 50mm f/1.4 and 50mm f/1.8 are interesting. As are his results for the 18-55 kit lens.
I have a Tamron 60mm f/2.0 macro which gives good stars out to the corners at f/2.5 but the illumination drops off substatntially and would need to be used with flats. I wasn't expecting that.
One thing my testing really showed up was how bad the phase contrast auto focus is for astro. The contrast a/f is great.
The Samyang lenses seem to be excellent astro lenses if you don't mind fully manual.
Steve.