"Mucho Gusto" or "Encantada de conocerte" or just " un gusto" and if you are saying good bye you could add " Ha sido un gusto" ^^

Depend of the context. If you want polite then you can use the Jenifer explanation: "Encantado de conocerte" I or "Ha sido un placer" when you finish the conversation. If you want to be more colloquial this depend of the country and people and situation.

Hello, You could also say (mostly in an informal way): - Diga! - Bueno?

You mean like "nice to meet you"? well nice to meet you in spanish is : Un gusto en conocerte. But that's polite, i speak spanish and i use "un gusto en conocerte" when i being polite/formal. Or you can use "un gusto en conocerlo/conocerla" this one here is even more polite.