I need a good marimba concerto that could win a concerto competition.

Check out https://www.marimba.org/william_moersch.html. It gives a good list of concertos. If you really want something tough, I would go with anything under "XVIII. Modern concerti." Though I have not played them or seen the scores, my PERSONAL recommendations are the Bennett, Klatzow, Nuyts, or Vinao. Zivkovics two concertos are pretty cool too.
Definitely get a second opinion before going nuts and buying a bunch of scores. Good luck!
~Behzad