The best place for AP (Access Points) What you need to level up is downtown areas. You don't necessarily need to have portals from the opposite faction, Grey's (or neutral) Portals are great for "Microfielding".

This is a great way to gain AP quickly". Microfielding is when you take over neutral Portals that are close together and link them together creating a small or Microfielding.

You get AP for converting a neutral Portal, You get more AP for linking Portals together, and even more AP for creating "Fields" Thank you @Fourtrys for this piece of advice!