I highly, highly recommend playing the whole series in order. They each stand alone, yet if you play them all you get to see the whole history, to watch the power of the Geneforge spread like a virus.
Some folks have their favorites...I personally like GF4 the best, but that's coming from a guy who played and enjoyed them all. GF4 may be the culmination, but I recommend playing them all in order to get maximum enjoyment.
So, really, begin with GF1.