(apologies for the semi-necro)
Well, it's a bit of a tossup, but I'd have to give my vote to G4, for several reasons: better graphics (a minor consideration), much improved interface and mechanics (IMO, anyway), and it does a better job of not smacking you over the head with the linearity than G3 does Also, it's the only game in the series that wants to play nice with my new comp
Although I did enjoy G1 very much as well, and let's face it, you've GOT to love a game where one of the possible endings basically has you becoming an insane, all-powerful demigod and enslaving all of reality
( EDIT: I speel gud )