I guess You should learn Modern standard. It’s the mother tongue of all dialects.

That’s a tough question for Arabs to answer. I guess English natives who’ve learnt Arabic would be able to tell. But, logically speaking, French is easy to learn for English natives, Tunisian, Algerian and Moroccan dialects all have lots of French words. Therefore, I’d argue that those dialects will be easier to learn. The real question, though, is how much those dialects will serve you with an Arabic community? Depends on that community. But, If you don’t know then go for Egyptian as it’s the most popular and widely understandable among Arabs. Levantine comes 2nd.

Depends on why you are learning the language

I would say Egyptian and Levantine dialect because it's understandable in most of the Arabic countries

I would recommend checking this video out: https://youtu.be/CugKpLZKJXg As it might help you decide 😊