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I believe that Earth and Air are the most versatile. Earth provides Earth Eye, Entangle (even Fire doesn't have such a powerful destructive spell, except for Armageddon, but I don't really like it), Portal to Town, Star Rain, Slow, and Resurrection. Air provides Air Eye, Lightning, Chain Lightning, Haste, Astral Gate, and Fly. I really like that combining the information gathered from the Earth Eye and Air Eye practically gives you knowledge of the entire map.
