Why Touma is considered only a level 0 esper when he's clearly so powerful and was able to defeat the most powerful level 5 esper? Toma has the "imagine breaker" which is regarded by many as a very powerful hability, moreover he was able to defeat the most powerful esper and one of the most powerful magicians, one who was capable of turning into reality what he thought.

Why Toma is considered only a level 0 esper then?
For whatever reason, Toma Kamijo's Imagine Breaker ability is not considered an esper power, so according to Academy City's ranking system he's a level 0 esper with no known abilities.  It's also not seen as a magic ability so he's not considered to be on the other side of the technology/magic divide.  The true nature of the power hasn't been revealed.  Its origins are unknown and the series hints that even Toma doesn't know what its fully capable of.

Toma's ranking is confirmed through the annual System Scan tests, which corresponds to the mandatory physical exams that Japanese students have to take every year. The real life exams measure things like height and weight, but the System Scan tests measure student's esper abilities and these tests have always shown that Toma is only a level 0 esper. 

The series also tries to portray Toma's encounters against powerful level 5 espers as a David versus Goliath situation.  A mere level 0 who has the courage to take on the most powerful level 5 espers in Academy City in order to protect his friends.  Given how powerful his Imagine Breaker ability is, this lacks some credibility, but it doesn't stop Toma claiming this himself as he punches someone in the face.