元気です　is formal. 元気だ　is informal. Although it is grammatically correct, I haven't heard it. Young guys say 「元気だよ」 instead. 元気　is grammatically incorrect, but we use it in a casual occasion. Young girls particularly say like this, Girl A　「げんきぃ～？」（question) Girl B　「うん、げんき」 (answer) Although I'm not a young girl, I sometimes say this to my friends. haha!

です is formal だ is informal 元気 is a な-adjective so you need to put ですor だ after it. Without either だ or です ending, it's an incorrect sentence.

If you are asking when you want to answer, you can say 元気 or 元気だ to your friend, but if you wanna answer to the person elder than you for example, better to say 元気です な is for adjective as you know, so we can say 元気な人, but this word な depends on adjective you use. Do I get what you mean??