The following is a list of known directories to add to your IDE for effective code completion using AVR-based Arduinos (version 1.6.5).

{path-to-arduino}/hardware/arduino/avr/cores/arduino/
{path-to-arduino}/hardware/arduino/avr/libraries/EEPROM/
{path-to-arduino}/hardware/arduino/avr/libraries/SPI/
{path-to-arduino}/hardware/arduino/avr/libraries/SoftwareSerial/
{path-to-arduino}/hardware/arduino/avr/libraries/Wire/
{path-to-arduino}/hardware/tools/avr/avr/include/
{path-to-arduino}/hardware/tools/avr/avr/include/avr/
{path-to-arduino}/hardware/tools/avr/avr/include/compat/
{path-to-arduino}/hardware/tools/avr/avr/include/util/
{path-to-arduino}/hardware/tools/avr/lib/gcc/avr/4.8.1/include/
{path-to-arduino}/hardware/tools/avr/lib/gcc/avr/4.8.1/include-fixed/

With this list, you'll want to do one of the following:

• If your IDE lets you configure custom toolsets and you've configured an Arduino toolset, add the directories to the Arduino toolset's list of directories checked for code completion if it lets you.¹⁾
• If your IDE is workspace based and you use separate workspaces for Arduino development, add these to the list of the workspace's additional directories to check for code completion.²⁾
• Add the list to your IDE's global list of of additional directories for code completion—which means the directories will be parsed for every project, not just Arduino ones.
• Add the list to the IDE project's list of additional directories for code completion. If you take this approach (or if it's your only option), then you'll have to repeat this for every Arduino project you start.

You should add any of the following libraries that ship with Ardunio that your project uses to the IDE project's list of additional directories for code completion:

{path-to-arduino}/libraries/Bridge/src/
{path-to-arduino}/libraries/Esplora/src/
{path-to-arduino}/libraries/Ethernet/src/
{path-to-arduino}/libraries/Firmata/src/
{path-to-arduino}/libraries/GSM/src/
{path-to-arduino}/libraries/LiquidCrystal/src/
{path-to-arduino}/libraries/Robot_Control/src/
{path-to-arduino}/libraries/RobotIRremote/src/
{path-to-arduino}/libraries/Robot_Motor/src/
{path-to-arduino}/libraries/SD/src/
{path-to-arduino}/libraries/Servo/src/
{path-to-arduino}/libraries/SpacebrewYun/src/
{path-to-arduino}/libraries/Stepper/src/
{path-to-arduino}/libraries/Temboo/src/
{path-to-arduino}/libraries/TFT/src/
{path-to-arduino}/libraries/WiFi/src/

You can add all of these to the default configuration, but giving your IDE more directories to parse will slow it down.

In addition to the above, you should add any libraries you've downloaded into your `{Arduino-install-directory}/libraries` and used by your project to the IDE project's list of additional directories for code completion. These will typically be of the form:

../libraries/{name-of-library}