Physical Strength:
* Powerful
* Robust
* Stalwart
* Vigorous
* Muscular
* Sturdy
* Hardy
* Resilient
* Durable
* Tough
Mental Strength:
* Resilient
* Determined
* Tenacious
* Brave
* Courageous
* Resolute
* Unwavering
* Indomitable
* Unbreakable
* Steadfast
Emotional Strength:
* Forgiving
* Compassionate
* Empathetic
* Kind
* Loving
* Generous
* Patient
* Understanding
* Resilient
* Optimistic
General Strength:
* Powerful
* Forceful
* Impressive
* Influential
* Dominant
* Authoritative
* Effective
* Impactful
* Remarkable
* Exceptional
More nuanced options:
* Unbending
* Unyielding
* Invigorating
* Fierce
* Bold
* Assertive
* Confident
* Capable
* Skillful
* Proficient
The best choice will depend on the specific context you are using the word in.