I personally find Gundrak and DTK quite good for getting into heroics since they allow you to skip certain bosses.
The first two bosses in UP are quite easy as well and the first one drops some really nice caster DPS pants.
Same goes for HoL, the first two bosses are not that hard, Ionar however can be really annoying.
CoS is all about being patient and just completely ignore the timer.
And ofc good all UK, if you manage to beat the first boss the rest isn't really that bad.