Viszlát is very common and is more formal than szia. The longer, even more formal version of it is viszontlátásra.
As spoken above, it is more formal, and in telephone we say Viszhall or Viszonthallásra.
Szia is not only informal but relatively new. Never use it with older people or those with higher status! Stick yo using it with your peers and kids.
I remember young school kids hopping off the bus shouting what sounded like a short version of köszönöm - "kösz!" to the driver. Is this an even more informal way to say "thanks", perhaps a little slangy?
Köszönöm is the "original", köszi and kösz is the shorter and more informal way. I prefer köszi, but when you want to thank something to a stranger (adult), always use köszönöm.
What is the difference between köszönöm and köszönjuk? I've read and heard both of them but I don't know if they are the same or should be used in different contexts