Huh, according to GameFAQs, Terranigma was released in Europe, but not North America.
Act Raiser is a rather unique game. It's half side-scrolling action and half town-building simulation. Overall, it was very well done and worth a play if you're into either genre.
Soul Blazer is the ancestor of Dark Cloud. It's a zelda-like action/adventure that throws in recreating worlds as you advance through dungeons and kill monsters. Its plot is thinner than IoG or Terranigma, but the game is somehow appealing.
Terranigma is the conclusion to the series. It takes place in earth and much of the game has the hero helping miscellaneous inventors complete their inventions, such as the hamburger or telephone, or reviving the world in other ways. Its plot is surprisingly deep and intriguing, making the game very worthwhile to play.
Of these four games, IoG is by far the weakest link. Terranigma has an engaging plot, while Act Raiser and Soul Blazer both have addictive gameplay.