which service pack I should apply first after installing sql server 2012 enterprise edition either sp1 or sp2 ?
Thanks in advance.
The Best Practices, that I've been following would be to start initially with SP1 and will check how it behaves and then will upgrade it to SP2.
Depending upon the application and other various parameters its better to first implement such upgrades on DEV/TEST server first and monitor the behavior accordingly. Once everything goes well you can decide how to apply SP's on the production box.
Also, note that Cumulative updates are the one which are truly cumulative. i.e if i need to install SQL server 2012 CU9, i can go like installing SQL server 2012 RTM , then SP2 and then directly CU9, instead of installing CU's from 1-9..
You can refer here for the SP releases.