Generally, choosing a good alignment makes a good character. Sorry, it’s been a very long day and my brain is mush.
I tend to think that the most important element of a good character is playing something you really want to play and picking some elements for the character that you really identify with, either because it lets you play a favourite character from a TV show or book, or because the character has an agenda you strongly believe in.
It’s important to feel comfortable with your character and enjoy the elements that make them unique, if you have a personal hatred of violence then playing a bloodthirsty gladiator might not be for you. It’s about finding the nugget within the character that you can run with and use to make your character stand out.