I agree with Michael below.  The original TR is hands down the best in the series.  Each successive TR game, though it may have enjoyed better graphics, sound, cut scenes, etc, also became progressively more difficult  and borderline unplayable.  TRII was fairly strong, although it wasn't as  good as the first.  But III and IV introduced problematic frustrating  levels that sent one running to the internet (or, for the more foolish, to  the nearest store to by the official Eidos hint guide).