It depends on several factors like your knowledge, your age, what you are interested in, etc...
If I asked "What is the most exciting game to play?", my father would say chess, my brother would say tennis, and my little cousin would say tic-tac-toe.
Maths is a very great and important study for life. i said important for life because every one uses math knowingly or unknowingly. in the world everything is a maths including human behaviors. oops i think i am going beyond your question. my answer is learn as much as possible all the fields in maths are interesting. but what ever field you choose, start from the basics.
Everyone knows that the most exciting topic in mathematical physics is stuporspace, which includes such techniques as Chevrolet Cohomologies, off-Shell Petroleum Derivatives, Non-Abelian Wind-Mills, Telephone operators, and the McDonald-Hamburger nullity relations!