which ultra high end wide angles have caught your attention? I've been taking a look at the Zeiss 21mm Distagon and the Leica 18mm Elmarit, any others?
It's the Leica 19mm you want not the 18mm.
The Zeiss 21 is king for sharpness edge-to-edge. The Leica 19 is close but not quite as good on the edges but with a little better color but you have to break your mirror to use it on a FF Canon. The Zeiss can fit as is. I own both.
The Leica 28 is possibly the sharpest lens ever made in this range. The Zeiss 28/2.0 (not 2.
is very close and the Zeiss 35 PC Shift lenses are amazing. I own these too.
The Leica 15/2.8 is the best in its class. I don't own that one, I own the lessor Leica 15/3.5 which is basically the same design as the Zeiss 15/3.5 with possibly different coatings. These aren't as good but they are a good value and more common on the used market.