agree with a lot that has been written above. i think you need to find a class you enjoy as your first priority. that is if you ve just decided to take up a martial art for the sake of learning one. however if you have a particular reason i n mind i would suggest;
fitness = mma/boxing
sport = kick boxing/ tkd
style and flexibility = kung fu / wushu
fighting = krav maga
i know i missed some but this is just IMHO. i do kung fu and enjoy it very much.it`s improved my fitness, strength, co ordination and flexibility. plus my confidence.