Let me put in a word for the VisualWorks one...it's probably the most complete. It would let us share work with the VisualWorks and the Camp Smalltalk communities (and with Cincom itself). Cincom is definitely all the way behind it (they're even putting together some (hork) XML Schema stuff). I'm pretty sure they're open sourcing all bits, so we have an XSLT processer, XPath support, etc.
OK, then I get to advocate the exobox one :)
The exobox parser is a complete well-formedness, non-validating parser minus Unicode support - every obscure little syntax weirdness is handled, even if the result is eventually dropped on the floor. It is set up much like a SAX-style parser, in that what actually happens to the result of the parse is handled through overridden methods. There is a subclass provided that constructs a tree built from OrderedCollections (for nodes) and Dictionaries (for attributes). Also, the exobox parser handles some peculiar cases, including the very tricky Jabber one, where a entire session is in fact one XML stream - in other words, the parser can spit out subtrees immediately as they close rather than the entire tree, without blocking on waiting for the next token.
Plus it's released unambiguously under the Squeak license and doesn't require convincing anyone to make Squeak-friendly changes. It's not, however, a speed demon, but that's fixable.
-- Duane