You really need to find a fitter with a launch monitor, club head speed is a factor but the other factors are just as important and sometimes more important so my suggestion is to find a fitter

Shen,

Without knowing your ball speed and launch conditions it's hard to recommend a shaft for you.  My recommendation is to take the time and go get fitted with a Titleist fitter who has a launch monitor.  They will be able to get you into the right 910 head model and shaft that will maximize your drives.  Based on 120 mph club speed which is really good, your probably going to want to go with a little stiffer and lower torque shaft.  As far as the stock shafts,  I have the 910D3 9.5 with the Project X shaft and absolutely love it.  A lot of guys also like the Aldila RIP as well.  Those would be the two stock shafts I think you will like.  But go get fit by a professional to make sure and get the right head/shaft makeup that's best for your swing.  Best of Luck! 

Thank you!

OK so you hit it hard but that's about all we know.  hard to pick a shaft just based on swing speed.  I guess you need a relatively heavy shaft to give you feel.  Other than that, it is best to get fitted.  I'm more like 105 mph and love the kali'i stiff.  In XS, I don't know what to recommend. 

Where do you live so perhaps Titleist can tell you where to go for a fitting.

When I got fit, they got my trajectory just right and my spinrate just right.  It added 15 yards on a calm day and 30-50 yards into the wind.  I don't think that there is that much difference in the 909 vs the 910 heads so I think it must be the fitting.

Play like the pro's, get your clubs custom fit.

--Jerry