I was talking about this in another thread but I think I should bring it to this thread.
Admittedly Gaspard might a better choice for the immediate situation of dealing with Elder One, but its not true for the long run. After extensive information about how Orlais works and its history, Gaspard will become a failure. He wants to remove the "game" and influence if politics from Orlais, but we all know he can't. He may enforce it for a while but he will die someday or simply lose power, then all of it will come back immediately. He will quickly lose favor and his reign will solely depend on his army. Guess what? The army is fed by favors.
Celene is not without flaws either, but its not like there is any other choice. Based on epilogue endings and what they describe, the most stable ending for Orlais is Celene and Briala united and Gaspard executed. There is not a single mention of "looming threat" in this one, but in all other Orlais endings its mentioned.