I'm a big fan of playing computer games and GTA is one of the series I like to play the most. Some of the Patch version which gives the best performance in GTA V PC are mentioned below:-
1. Patch version 350.2 [5/1/15]
2. Patch version 350.1 [4/30/15]
3. Patch version 335.1 [4/21/15]
4. Version 323.1 [Release version 4/14/15]
But as also stated above "Version 323.1 [Release version 4/14/15]" gives the best performance among all the other.
Apart from this, you should kill unnecessary processes from task manager which can be accessed by pressing ctrl+shift+esc. Moreover, type "msconfig" in run (without the quotes) and hit enter. Now go to startup and disable all the unnecessary programs plus go to services and disable every service which do not contains "Microsoft" as manufacturer. This will surely boost the performance.