Basically every Bejoijo mod. He is one of the best if not the best LSPDFR modder.
Stop The Ped is amazing and it has so many features. So, so so many. Includes stopping people, moving them, drug/alcohol tests, drug tests to do on scene with your car, searching vehicles and peds, arresting peds, stopping traffic, calling prisoner transport or taking suspects to the station yourself, calling animal control, coroner, tow trucks... and taser lasers in the latest update)
Ultimate Backup is also fantastic. You can call backup and they will stay on scene, you can tell them do stuff for you (with Stop The Ped), you can get partners, get realistic traffic stop and felony stop backup, you can dismiss certain officers or all of them, and during a pursuit you can request spike strips or roadblocks.
He also has 2 callout packs and ALPR Lite and SpeedRadar Lite, both of which are great to use for traffic related things. Compulite is also a great police computer.
https://bejoijo256.wixsite.com/bejoijo/home/
As for other mods:
There are a ton of callout packs... you can choose which ones you would like. I use both of Bejoijo's callout packs, Assorted Callouts, Super Callouts, United Callouts, PeterUCallouts (this one is amazing). Traffic Policer is a plugin with ambient traffic related events, as well as a few calls, and other features (traffic cones, barriers).
Police Smart Radio is also pretty nice to have. You can quickly run plates and ped names. It has integration with lots of other mods, like Bejoijos for panic buttons for example.
A few other mods are good to have, like Force a Callout, Rich's Police Enhancements...
I know, there are a ton of plugins alone. There are even more to choose from but these are some I would recommend to first time players. Hope this helps! Have fun playing LSPDFR!