the best cpu you can get without upgrading your motherboard is i9 9900K (but first you would need to update your bios to the latest version)
is it worth it ? the answer is no , not really , it is very expensive cpu (currently 490 dollars on amazon) and it is already outdated
(also your motherboard doesn´t really have the best VRM for such power hungry processor) .
what is the rest of your system and what do you expect from upgrading your cpu - do you use your pc mainly for gaming or work ?