Celene, obviously. Who in their right mind would pick Gaspard? Lets examine the facts:
Everyone in Orlais rises to power with underhanded tactics. Celene and Gaspard are no exception.
Gaspard's honour didn't stop him from ambushing weary soldiers or smuggling mercenaries into a peace-talk for a coup.
Gaspard, despite being a warleader, isn't enough to save Orlais. As we see in the future, even with Celene dead he isn't enough to protect Orlais.
The greatest ancient civilizations and the ones who endured are the ones that focused on culture and education (Celene) not war and conquering (Gaspard)
Celene has proven to be an powerful Empress, uniting Orlais with the aforementioned Culture+Education+Diplomacy. Gaspard can only lead with war.
The Inquisition is here to restore chaos, not create it. Putting Gaspard on the throne is asking for trouble.
Even if you avoid a war with Ferelden, you take away Gaspard's only way to lead. Its the only thing he is good at.
Orlais enjoyed a Golden Era for years until Gaspard's selfish uprising.
I mean, really. The only time I see anyone supporting Gaspard its because they point the finger at Celene and say "LOOK! She did bad things!" Of course she did! You need to make tough decisions when you're a ruler. Bull said the Qunari (The most pragmatic culture out there) choose leaders from those who can make tough decisions, not those who want to lead (Gaspard) or those who are inherently powerful. Lets not forget, its Gaspard who started the war, not Celene. Everyone who dies is because of him.