What's the best way of tracking a lost iPhone?

Learn more about how use the Find My App.

Find My - Official Apple Support

Then look at this Apple article and my additional advice below it:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support (UK)

Tracking lost or stolen phones

The 'Find My' tracking is to help you discover if you left your phone in the office or the restaurant. Lost or Stolen phones are rarely ever seen again. They are often processed invisibly by organised crime gangs.

Risk mitigation

Get your provider to 

Cancel your SIM.

Send you a replacement SIM on same number.

You can then put the SIM in another iPhone to get the codes you will need.

Phishing warning

It’s possible for a thief to obtain your phone number particularly if you did not have a PIN set on the SIM.  So do not respond to texts askimg you to sign into your account to get your phone back! They are NOT from Apple. 

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

you cant track them by serial number or imei

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

though the police if they have a warrant can force carriers to triangular track devices using the IMEI they only used it in of missing persons I believe

Should we assume that you previously enabled the Find My option on your phone? If you did, and the phone has ample battery power and an Internet connection where it is located it will show up on a map when you enter the Apple ID and Password associated with the phone.

www.icloud.com/find

Be on the lookout for any messages that you might receive from "Apple" or "Apple Service" or even the "Police" notifying you that the phone has been found. These are SCAMS that are trying to trick you into revealing your Apple ID and Password so the bad guys will have full access to the data on your phone.

Apple and the police will never send messages like this.

Realistically, the phone is gone, so you will need to protect yourself.

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support