The first and second games are the best in my opinion. The first one has a great story, and is a wonderful game. The second has improvements, such as additional spells and improved inventory, while still having a fantastic story to it as well, making it a worthy sequel.
I chose the first game. Although it lacked many things the others had, it still was an amazing game, and left a great impression on me, possibly because it was the first in the series. One of my favorite games, and part of one of my favorite game series.
The third was probably my least favorite. It is a great game, and I am not saying that I dislike it at all. I just love it a little bit less than the others. It had offered some nice new things, such as the ability to craft things, as well as some new graphics, though when I first played it, I missed the classic ones. Perhaps the reason I put this game least is that I lost my save when getting a new hard drive, and I was on the Isle of Spears, almost finished the game, but I never got to before my hard drive broke.
Again this is a great series, I have not mentioned even a slight fraction of what these games have to offer, but I don't want to bore anyone with a post that is too long. All of these games deserve to be played, and choosing one isn't right. Geneforge 5 isn't an exception either, it wouldn't have hurt to include it.