This is one of those unanswerable questions like "what's the best motorized vehicle?" :-d Do you want great gas mileage, acceleration and maneuverability? You probably need a motorcycle. Do you want to deliver home appliances like refrigerators and washing machines? You probably need a truck. Do you just want to commute to work in all weather conditions? You probably need a car, or a SUV, and so on.
I've also got multiple answers to the "best all digi G" question, but it again really depends on what the WEARER wants!
I really like solar/atomic models, but for people who don't go outside much, or live near the Arctic Circle, or don't live within range of the six country's radio transmitters, that's not going to be a "best" feature for them.
I used to think the GW-6900 was one of my best G's -- iconic looks, solar/atomic, and it fits nicely on Casio strap adapters, which is nice if you don't want to worry about losing the watch if one springbar breaks. As a side benefit, it makes it really easy to change the look of the watch by changing the strap in seconds, without tools.
Before that, I briefly thought the GW-2310 was one of the best G's -- solar/atomic and (like the GW-6900) it could be purchased for about 60% of the MSRP.
After the GW-6900, I thought the best almost-G's were a couple of Casio Pathfinders -- ALMOST as tough as G-Shocks, they feature cool outdoorsy gadgets, and they also have very large, easy to read displays. One of mine is also solar/atomic, so for me that's even better! Unfortunately, the watch cases themselves are relatively large, so they don't fit well under coat sleeves, but for me they're one of the best choices six months out of the year!
Currently, the best all-digital G in my collection is a customized Multiband 6 MTG-M900. The module has the largest "main display" digit size of any G-Shock, it has all of the most common features (including a 24 hour stopwatch and 24 hour countdown timer), and I replaced the bracelet and stock bezels with a resin strap and more durable bezels.