Re:
From the aluminium stems I think the best stem is made by Syncros. The welds are perfect and form follows function in a very visible way. I think the design is very clear and honest. And the width and strength of the handlebar clamp is unmatched.
From the steel stems the Salsa and Ibis stems are great. From the titanium stems I'd take a Litespeed or Ibis, depending the build.