Now that Firebase White is no longer as easily farmed, I bring you an alternative that is just as easy and quick (30-35 minutes per match): Firebase Rio/Reaper/Platinum! .
The Geth Infiltrator is indisputably the best killing machine in the game - just don't get shot. :) Here is the ideal build and why: BEST BUILD - 4/6/6/6/4 Q:.
*UPDATE MAR 1 2013* - This farming method is no longer reliable since the last map changes and Piranha nerfs. It can still be done, but it is a little.