What are the criteria for choosing a Rabbi to fulfill what we are taught in Pirkei Avot (1:6): “Aseh l'cha Rav” [You shall appoint a Rabbi upon yourself]?
To the Questioner,
The idea of “Aseh l'cha Rav” is mentioned twice in the first chapter of Pirkei Avot. This can be viewed as a hint that a person needs two types of Rabbis. A person must certainly have a Rabbi who renders him halachaic psak (decisions). However, in addition, it is very important to have a Rabbi nowadays for guidance and direction (hadrachah) as well, to be able to ask questions specifically about how to progress in one's relationship with Hashem. [Often both of these directives can be accomplished by one and the same Rabbi].
The criteria by which to choose a personal Rabbi are described in Sefer Meor Veshamesh. He sets forth two specific criteria:
1) That all of the Rav's behaviorisms are consistently according to Torah law, and that he is never lenient at all in regard to any halacha – whether it be Torah law or Rabbinic.
2) [You can sense that] within his heart is a burning passion to unite with Hashem, and his desire is apparent through his actions.
Of course, in addition to these two criteria, the Rabbi that you choose for long term should also be someone who you feel understands where you're “coming from” and who has some experiential connection to your background.
This is necessary so that he can correctly gauge your situation in regard to any halachic or hashkafic sheila (question) which you may have. Only someone who can properly relate to the background and particulars of your situation will be able to answer your questions in an appropriate way that is balanced and in proportion according to your true reality.
For example, if an English-speaking Ba'al Tshuva who just arrived in Israel six months ago, decides to accept upon himself as his personal guide a Rav from Meah Shearim who has no exposure to any lifestyles outside than that of Meah Shearim, he is likely to receive the wrong hadrachah (guidance) for himself – and maybe even the wrong halachic answers because they are not in proportion according to his true reality – when seeking direction from a Rav who does not truly understand his background.