It would most likely be HP Ground, as it allows TF to hit stuff like Heatran and Tyranitar super effectively, which would otherwise wall it. Only problem?The opposing Talonflame used Hidden Power!
A critical hit! Blissey lost 3.1% of its health!
... I can't believe that Talonflame didn't have any better move (and it was in 1v1, so Blissey wasn't going to switch)
4 SpA Talonflame Hidden Power Ground vs. 0 HP / 4 SpD Heatran: 132-156 (40.8 - 48.2%) -- guaranteed 3HKO
That means TF needs a Life Orb to 2HKO offensive Heatran (not to mention some of them have Air Balloons). And Tyranitar takes even less. So it's still stupid.
Edit: Forgot to mention I saw an HP Ground Talonflame myself many moons ago and posted pretty much the exact same logic ITT. (I switched Gyarados into it, which is how I found out.)