Of the 3 DBMSs you suggest, I would lean towards Oracle or Informix as a starting point, especially if you're just learning, since both support (and extend) the ANSI SQL standard whereas M$, quite typically IMO, have seen fit to modify the ANSI SQL standard in SQL_Server.
That being said, as has been said previously on this thread, MySQL would provide a better (aka far cheaper:-) starting point - from there you can, if funds permit, then go on to Oracle/Informix...
A user level that continues to overstate my experience :-))