I now believe the Tsunade's kind of strength would be Might, and is completely different the Strength in discussion here.
That said, strength automatically means many things:
* If you're strong, you have muscle power.
* If you have muscle power, you're also fast.
* And you have better protection from attacks taken head-on.
That means, the one with the strongest physical strength would also have:
1. Strong punches.
2. High speed, but not necessarily agility.
3. High endurance.
Potential Contenders:
Might Gai: very strong punches, high speed, high agility, and above average endurance.
Rock Lee: strong punches, high speed, high agility, and average endurance.
Naruto: very strong punches, highest speed, highest agility, and highest endurance (without self-healing).
Tsunade: strongest punches, average speed, high agility, and above average endurance (very high, if we consider self-healing).
Sakura: very strong punches, average speed, high agility, and average endurance (high, if we consider self-healing).
2nd Raikage: very strong punches, very high speed, above average agility, very high endurance.
Hashirama: very strong punches, very high speed, very high agility, very high endurance. (all due to sage mode)
Considering all that, we can rank them:
1. Naruto
2. Hashirama
3. 2nd Raikage
4. Might Gai
5. Tsunade
6. Sakura
7. Rock Lee