Can be a tough decision. Depends on what you value most. You mentioned exploration, group benefit and survivability, so lets take a look at those three.

For exploration I would suggest healer or bard. The healer offer better survivability for solo and group exploration but the bard offer more utility. The ability to calm down aggro mobs and put groups of mobs to sleep can be quite handy. Also, if you ever end up in a group where your healing isnt desperately needed, the bard can deal some decent damage.

For group benefit, the transmuter is looking quite good. They can turn a decent tank into a great tank and help turn tough mobs into harmless puppies. But they need to wary against mobs that use breath attacks since they cant offer their group extra damage reduction. If you just want to help a group bring down a tough mob quickly, transmuter is the way to go.

Survivability. Here the healer shine. It is hard to compete with sanctuary. The other two classes can offer some extra nice things through the use of talismans which is not something to sneeze at but as a whole the healer gets the job done.

If you take other things into consideration, such as leveling and pk, my vote would go for the bard. With parry, dodge and shield block, they make pretty good tanks. When they get acapella they can even heal themselves while tanking without having to switch to an instrument if it becomes necessary. The ability to put groups to sleep is handy even here, for pk situations or when you try to rank on mobs that tend to bunch up. They wont be as good solo pkers as the transmuter though, but also have less natural enemies so if you dont aim for pk, the bard is the safer bet. And to top it off, the bard doesnt need empowerment like the healer does. That alone makes it the better choice imo.