Aurum Vale can be tough on a new healer if people don't grab fruit.
Even with the changes the frog hallway in DD can be dangerous
Qarn first boss still kills an unsuspecting new person now and then, but more tanks interrupt this now.
Chimera in CC is probably most common boss for a death or two, but rarely a wipe

At level 50, probably Pharos Sirius is where I see the most deaths. Both first and second boss I've seen wipe groups
Keeper of the Lake final boss kills the most people on the MSQ path. Sadly I bet they change that one for trusts next patch