Which brings us to the strength. Naruto cannot achieve his dream if he is not strong, but what is strength? How is it measured and where does it come from?
There are two kinds of power that make up strength: physical power and spiritual power. Physical power encompasses all jutsu, talent, and special abilities as well as stealth, speed, and actual muscles. Spiritual power is about the inner character of the fighter—the intelligence, ambition, heart, discipline, principles, and perseverance that leads one to understand battle and drives one to fight in the first place. The measure of true strength is a combination of physical power and spiritual power.
A ninja's physical power can be difficult to measure.
There is taijutsu, genjutsu, ninjutsu, five (maybe six) elements, different chakra levels, different control over charka, yin and yang charka, etc. There are also obvious factors like age, intelligence, speed, size, stamina, muscle strength, etc. Most recently, we've seen the addition of Sage Techniques. Etc and so forth.
You can get a rough idea of a ninja's physical power based on his or her missions (remember that "D" "C" "B" "A" and "S" apply to missions, not ninja rank), but it's impossible to compare one ninja to another and know for sure who would come out on top in a fight. This is because there simply aren't enough common denominators. Rather than power levels, ninjas win battles by tactics.
There are factors to influence favorites, of course. There are special abilities like Kekkai Genkai or Bijuu. Some ninjas come from fearsome families or have had fearsome parents, teachers, or trainers. There are also statistics in the data books, which are helpful or basic comparisons. Most of the time, the more intelligent fighter has an edge over the stupider fighter, and the same with the faster fighter to the slower, or the more experienced fighter to the inexperienced,...