The widest I have in manual focus are 24mm lenses, ultimately anything wider than that and you dont have to worry to much about focusing as much of everything is already in focus with a very short throw.
I have a Tokina 19-35 3.5-4.5 AF, it's in a ton of mounts. It is a good performer and super inexpensive. I got mine for about $35 as a rebranded quantaray(ritz) model. It goes for about $70 on the bay as the tokina model. There are a ton of reviews about it online.
The other superwide I have is the Sigma 14mm 3.5 AF. It is a bit slower than the newer 2.8 model, but its not a big deal. You have to cut your own filters and slot them in the back. It is a rectilinear and one of the widest you can get. It goes for about $200 on the bay.