Rock Garden is by far the easiest of the Hard dungeons. By FAR. (Spoons!)
While this is true if you've unlocked lots of stuff, if you have yet to unlock your T3 wizard tower to prep ENDISWAL or acquire Binlor's altar it is much
more difficult. Three bosses is pretty daunting for beginners, and they are usually placed in such a manner that there's little to no exploration left by the time you find them, and the level has a tendency to block you off at regular intervals with plants and dead ends or monsters. Once you get momentum going, the spoons and extra gold income from boss kills will make things much smoother, but getting that momentum if you don't have high-end preps or the right altars unlocked is not trivial.
Overall for a beginner with low unlocks, I'd say Rock Garden is now the hardest
of the second-stage dungeons.
I'd say either Hexx Ruins or Havensdale Bridge is the easiest of the hard-difficulty dungeons right now. Havensdale is smooth sailing if you can get past the toll troll without having to pay him off (easily doable with a mid-battle level-up), and Hexx Ruins just requires you to find a way to deal with Revenants. If you can clear out the level of regular monsters, Stheno is a complete and total pushover.