Physical Strength:
* Powerful
* Muscular
* Robust
* Sturdy
* Mighty
* Vigorous
* Hardy
* Forceful
* Tough
* Durable
* Stalwart
* Brawny
* Athletic
Mental Strength:
* Determined
* Resolute
* Courageous
* Bold
* Brave
* Resilient
* Tenacious
* Unwavering
* Unyielding
* Firm
* Steadfast
Emotional Strength:
* Passionate
* Intense
* Fierce
* Ardent
* Spirited
* Committed
* Dedicated
* Passionate
* Intense
* Fierce
* Ardent
* Spirited
Other:
* Potent
* Effective
* Efficient
* Solid
* Stable
* Durable
* Reliable
* Sound
* Substantial
* Significant
The best synonym to use will depend on the specific context of your sentence.