Melee LB: Used in most content, so be ready to use it a lot. Note that some people might take offense to you using it before the final boss (any of the LBs really, but Melee is most used.), but if you are able to get it back up to the full bar before then it doesn't actually matter that much.

Caster LB: Second most used, and any content that would have Melee LB used will have this used in its place if there is none. Some situations also see Caster LB being better than Melee (EX: During Copperbell second boss, Caster LB can pretty much immediatly wipe out the final little bits at the end, or in WoD, a LB3 caster can take out all three clouds that need to be killed if it starts before the barriers come up.)

Healer LB: Only used at LB3 for its Res ability. Take note that it rezzes people where they died, so it isn't as useful in certain situations like Titan. Bard also gets this one but its being changed for Heavensward.

Tank LB: Most underutilized, it sees the most work on new content...got to use LB3 version twice on the first day of WoD, and another time a week later...only group that survived that one, that was fun. Also apparently good for t13 at a certain point, but I haven't gotten into FCoB.