There are LOTS of good rabbis. I would generally try to find one in your own community because he will understand the local customs, which do differ. And find one with which you are comfortable sharing about more than just purely halachic issues, because a rav should not be merely a halachic resource but also a spiritual guide. “Aseh l’cha rav” is an important mitzvah. Good luck!
There is a thread about “ask a rabbi” websites on Stack Exchange (Mi Yodea), which lists various sites. The only well-known posek I know of who will quickly answer a question on any topic (through text or email) is the chardali (charedi-dati-leumi) posek Rav Shlomo Aviner. But like Charlie says, you should ideally find a good rabbi from your local community.
“The only well-known posek I know of who will quickly answer a question on any topic (through text or email) is the chardali (charedi-dati-leumi) posek Rav Shlomo Aviner. “
Rav Aviner has even published his SMS text message responsa!