Emphasis on Power:
* Like a raging river, he swept through all obstacles.
* He moved with the force of a hurricane, leaving nothing in his path.
* He was like a tidal wave, overwhelming everything in his way.
* He was unstoppable, like a runaway train barreling down the tracks.
Emphasis on Determination:
* His determination was like a burning fire, fueling his relentless pursuit.
* She was unstoppable, like a seed pushing its way through concrete.
* He was like a mountain climber clinging to the summit, determined to reach his goal.
* She marched forward, her resolve like an unbreakable chain.
Emphasis on Speed:
* He moved like a blur, unstoppable in his quest.
* She was a whirlwind of energy, her momentum impossible to halt.
* He was unstoppable, like a bullet fired from a gun.
More Creative Options:
* He was unstoppable, like the relentless march of time.
* Her ambition was unstoppable, like a wildfire spreading across the land.
* He was like a force of nature, unstoppable and unpredictable.
The best simile will depend on the specific context of what you are describing.