WHat is the best stiff aftermarket 910 d3 shaft?
The one that works best for you! Go to a fitter that has a bunch of shafts and try them out, the best one will most likely be different than your best friends. Depends on your swing.
Sorry to sound contrite, but it's not necessarily a particular brand & model, rather it's whatever shaft fits your swing best....I would recommend going to a certified Titleist clubfitter in your area and get fitted from one of the shafts on their demo cart...most fitting accounts have the new 913D fitting cart by now but the shafts for the 913D will also work for the 910D3....you might also want to compare your 910 D3 to a new 913 D3 at the same time to see if the newer model gives you better numbers (or at least good enough numbers to justify the upgrade $$$, keeping in mind the new 913D3 will be more forgiving than your current 910D3).
Personally I played the 910 D2 for a year with the RIP Alpha 60 with good results, and more recently am getting good results with an GD DI-6 stiff (warning - serious $$$ for this shaft from Titleist mated to a SF tip)....I just purchased the new 913D2 with the RIP Phenom 70 (along with the 4g weight chip) as that is what I was fitted into. Even though I have three shafts at my disposal for the 913D, I expect that when the 2013 season ends this time next year here in Nebraska that I will find the PHENOM is the "gamer" due to the fitting. I tried a couple other new shafts for the 913D (The new Mitsu Diamana Blue- and Whiteboard shafts) during my 913D fitting and they were nice but the numbers I got with the PHENOM were better than with either of the two Mitsu shafts.
Good Luck with whatever shaft you go with...