In addition to what Vlorsutes said, a lot of people fall back on Garuda if there's no specific elemental strength needed, because not only does she have a powerful Rage BP, but she can haste & heal herself. Leviathan can heal himself too, which for example can make him a better option than Ifrit for fire-based enemies.
Generally, fire-based = use Levi, earth-based = use Garuda, water-based = use Levi or Ramuh, wind-based = use Garuda or Shiva, ice-based = use Levi or Shiva, thunder-based = use Ramuh. Titan and Ifrit are kind of worthless, for the most part. Ifrit can be useful for fire damage sometimes, like against bombs & snolls for example, but I don't think I've ever used Titan for anything other than Earthen Ward.
If you're doing group play, another one that's very useful is Ramuh. Chaotic Strike & Shock Squall provide extremely powerful stun effects (more powerful than any stun in the game). Your group will love you for dropping a Squall here and there.
____________________________Pergatory (Asura, Kupo LS) WHM99 ~ SMN99 ~ DRK99 "If you want to know me, look inside your heart." -Lao Tzu