Mages are still really strong. Arcane is probably the easiest spec I've ever boxed with.
Paladin tanks seem to be everyone's favorite, but I'd shy away from using a Paladin for healer, they use a diverse spell pool and can be tricky to try to manage of their spells + Holy Power + cooldowns/procs
Shaman healers seem to be pretty popular and pretty easy to play in my experience, but I'd say priest is even easier.
As far as casters go, moonkins and elemental shamans aren't as strong as they've been in the past from a PvE standpoint. Warlocks are good damage but they're a little harder to box from what I've heard unless you're playing just warlocks. Hunters are another viable option. I'm not the biggest fan of shadow priests, they do good solo, not sure about boxing.

There's a lot more players in Zerg than LoB. A lot of the LoB people went to different games from what I've learned.