As I've posted previously, in the past 25 years I have used a variety of armor systems including the old military armor like the PASGT & IBA, integrated systems like the RAV and CIRAS, as well as a variety of plate carriers combined with various soft CBA. What is best is highly dependent on what threats you face, the type of operations planned, the environmental conditions and climate in your area of operations, your transportation method, and how long you need to wear the gear each day.
For LE uniformed patrol, where you are already wearing soft armor, just throw on a simple plate carrier preferably without a cummerbund, as this is the quickest way to up armor in an active shooter or other potentially violent situation where you may need protection from rifle threats.
Since the vast majority of LE officers already possess concealable soft armor they are wearing for patrol duties, when assigned to specialized tactical units like SWAT, one option is to simply throw on a plate carrier, perhaps one with a cummerbund and side plates. This is a simple, relatively cost effective solution that does not require getting a second set of soft armor for a tactical armor carrier (like the RAV, CIRAS, etc...). Many folks also find it more comfortable, versatile, and lower-profile.
As I have noted in a couple of other threads, I was issued and used a MAR-CIRAS for about 5 years; I found it more comfortable than our previous old school RAV's--particularly since the MAR-CIRAS had no cummerbund to sag or zipper to rip. Keep in mind that for LE use, the Land-CIRAS takes longer to get on than the MAR-CIRAS...
These days I much prefer a lower profile set-up, as typified by the Mayflower low profile carrier coupled with a rack type chest rig thrown over it. Compared to integrated armor carriers like the CIRAS or RAV, as well as plate carrier, a low profile armor carrier and chest rig has numerous benefits for me:
-- Greater range of motion
-- More comfortable riding in vehicles along with easier entrance and egress
-- Allows rapid change of load-out by quickly switching chest rigs, ex. assaulter, sniping, patrol, dedicated shotgun, etc...
-- Chest rig can be dropped and armor worn discreetly under a jacket or other clothing
Currently, I am really digging the AMI low profile carrier:
http://www.armoredmobility.com..._profile_carrier.php. It takes my soft armor and eSAPI large plates, along with 6x8 side plates.
Both Mayflower and AMI make MOLLE versions of their low-profile carriers for folks that want to add pouches directly to the carriers.
The AMI low-profile carrier is the most comfortable and versatile armor set-up I have ever used; there is NO way I ever want to go back to using my previous CIRAS/RAV or plate carriers--it even comes in Multicam...