The way you say 'goodbye' to someone depends on your relationship to them and how formal you want to be.
With a friend
Goodbye and bye are the most frequently used phrases.
Bye-bye sounds like what you would say to a child, if you say it to an adult it could sound childish or a little flirty!
Later / laters
See you later / soon / tomorrow / around
Catch you later
Have a good one / day - this sounds very American, British people don't normally say this, especially 'have a good one'.
It has been nice / a pleasure to meet you.
It has been nice to talk to you.
It has been lovely meeting you.
Well, I've got to be going.
I've got to get going now.
Do you have a (business) card?
Can we meet again sometime?
I'll be in touch!