As everybody answered, standard Arabic is understandable all around Middle East. Also, you have to note that everything in Arabic is written with standard Arabic. Only the spoken language differs. For the dialects, if you are going to work in a Middle East country, it’s obvious that you have to know some of the country’s dialect. If you are planning to learn only an Arabic accent, Egyptian is easer for English native speakers, and somewhat, widely known.

I think the most useful one would be Syrian (Damascus), since the Syrian drama has gotten a great boost in the Arab world during the past few years .. so pretty much everyone knows this dialect pretty well. But I would still say it's better and safer to learn the Standard Arabic as well, maybe at the same time if you think you can.

To be able to speak to most Arabs, one has to learn standard Arabic.

Concerning learning a dialect, that it depends on where you are planning to go or live where Arabic is the main language, i.e. Western Saudi Arabia > learn Hijazi, South of Jordan > Bedwin Jordanian...etc.

Egyptian Arabic dialect might be spread widely but it doesn't solve your problems every where, beside not being a one dialect (it has varieties) many don't understand it; standard Arabic does work fine with even non-Arab who are Muslim.

that's right forget about dialects and start to learn standar arabic which you can talk with everywhere in arabic countries..good luck