Closing Spell.
8d10+Int damage to a gigantic 121-square area (169 squares if you have the Resounding Thunder feat!), WHICH HITS ONLY ENEMIES, does half damage on a miss, AND you get to choose at time of casting whether it does cold, fire, lightning, or thunder damage, so you have a much better chance of hitting vulnerabilities or avoiding resistances if you know what you're fighting.
Blows every other Wizard attack spell away, including the level 29 dailies, and you get it at a ludicrous level 20.
They really weren't kidding when they called the path "Battle Mage".
But even that pales in comparison to the possibilities of the level 15 Ranger daily Blade Cascade.
Long and short of it is, you get to keep attacking infinitely until you miss. With a bit of set up from the rest of the team, stacking every possible buff to your attack rolls and debuff to the enemy's armor class for the round (and there are plenty of untyped to hit/AC modifiers available), you could very easily get into a "can only miss on a natural 1" situation with basically any monster. Throw in one or two (or five or six, if you're an Elf Kensai Deadly Trickster...) of the various "free reroll" abilities for when you DO get those 1s, and statistically, you're hitting some scary thing a hell of a lot of times in one action.
I haven't looked into all of the synergies and such that you'd want to make it optimal, or crunched any numbers, but it's obvious that it has potentially more power than anything else the Ranger can do, or anyone for that matter.