16 is not log term support, when you are planning to upgrade you should pick 13.1.x / 14.1.x / 15.1.x for LTS... you can upgrade to 15.1.2 and request engineer hotfix for the critical issues
BIG-IP software support policy
supported BIG-IP upgrade paths
regarding iRule, should monitor them after upgrade , i dont know if irules incompatibility between BIG-IP version
So is that mean version 15.1.2 is the best choice ?
and is it possible to upgrade directly ?
To upgrade from 13.4 to 15.1.2 , we have to upgrade from 13.4 to 14 then from 14 to 15 ? or we can jump to 15 !