In most cases, a neutral invoker will get you the furthest in exploration. Not only can they protect themselves and deal out immense damage in the right situations, but they can protect potential allies and fellow explorers.
In an exploration group, a transmuter is far from necessary.
Paladins are too restrictive. Simply put.
A good exploration group would consist of an invoker, healer and warrior. And unless you're Cabdru, you can usually only go so far alone with non-Anti-Paladins. Every class has its limitation. So if you really want to make it far in your exploration, pick one of the three classes I've listed, respectively.