There is not one most powerful class/spec class in DF. There is multiple and for different reason.
Guardian Druid is strong because it has good AoE option and Single Target options (both case with Lifestealing), as well with great mobility and good healing (but not powerful though).
Fury Warrior is strong because of the Whirlwind spam with Lifestealing enchant, and it has good tankiness thanks for being a plate user. It has good mobility too with Charge and Killing Machine Talent.
Arcane Mage is strong because of the scaling of both Arcane Explosion and Arcane Blast, which are boost via Mastery. It does lack healing, but has access to a shield and have good CC to help him.
Restoration Shaman is strong because of the base scaling of the damaging spells, as well with good mobility with Ghost Wolf and great, if not excellent healing spells. The downside are that the casting time is very slow, and if you spam your healing spell it will OoM you quickly.
Mistweaver Monk is strong because it has a bit of everything:
Strong scaling with Spinning Crane Kick as Arcane Mage (deal less damage though), and it's very powerful with Lifestealing, as Fury or Guardian (but it's still viable without compare of those 2);
Strong Healing Spell, as Restoration Shaman, without having mana issues;
Great Mobility with roll, Tiger's Lust (if taken), as everybody else;
If you take Rising Sun Kick, you have a strong single target option, as Guardian or Arcane;
A good CC ability, as Arcane Mage;
Great tankiness, as you're a leather user;
And you can spam your abilities without worrying about your mana, like Druid/Warrior
Last expansion, I can say from my experience that Mistweaver Monk was top 1, followed my Arcane Mage.
But now, the 4 first classes that I mentioned got buff in a way or another, which make them stronger than last expansion.
Mistweaver Monk is more like a versatile class/spec in DF; strong in everything but does not excel in one thing.
If question, don't hesitate!
Twink all day folks!