UFC belt and it's not close.
Why? Because it's a fucking choice, that Adults do and not all Olympic Gold medalist can get.
Most Olympic Gold Medalist are literally kids who have been training since they were kids......They never had a real choice, and had their whole lives controlled by their parents/coach.....Sorry can't really admire that......To me, that's worst than steroids(They are getting a huge advantage over 99% of humans), that's privilege at it's finest.
UFC belts on the other hand are won by adults who chose to get into MMA....and in order for them to become champs, they have to learn some new things/evolve.....That to me is why is much better/more valuable than a damn gold medal.
Not to mention, beating the shit out of people > Olympics