Of all the queens audiences saw during Game of Thrones, two of them were Sansa Stark and Cersei Lannister. Sansa Stark almost became queen when she married Joffrey, and eventually ended up as queen in the North anyway. Cersei Lannister was Robert Baratheon's queen, and eventually sat on the Iron Throne in her own right.
Each of them had qualities to make an awesome queen, and qualities that meant someone else may be better suited. Overall, who made the best queen out of these two?
Kindness: Sansa Stark
Sansa Stark was easily the kindest of the two and had the most mercy. Sansa wanted to help people and, although she could be a tad spoiled and selfish in her early years, seemed to genuinely have a good heart.
Cersei, on the other hand, was utterly ruthless and would destroy anyone to get what she wanted. She blew up the sept and killed the Tyrells and many others in a horrific blast of wildfire, watching from a distance and smiling. She didn't know the meaning of the word mercy.
Political Prowess: Cersei Lannister
When it comes to political prowess, however, Cersei was most definitely up there. Sansa had little to no political experience except for watching her parents operate, and started out clueless in the Game of Thrones. Cersei had years of manipulating people behind her, and learned all she knew from her very powerful father. She was, by far, the most politically experienced and knew how things operated within not just the Red Keep, but all of Westeros.
Stability: Sansa Stark
The most stable of the two, however, was Sansa Stark. A quote from the books from Jaime suggests that Cersei likes to think of herself as ice, but is all fire. Her temper got the best of her on multiple occasions, and although she could be cold in some regards, she wasn't someone who can remain still when something might have irked her.
A good queen generally requires more stability than Cersei had the ability of showing.
Experience: Cersei Lannister
In of experience, Cersei takes the cake. Cersei spent her whole life desperate to be queen, watching how things operate, and no one can deny that she was intelligent. Sansadreamed of being queen too, but she was thinking of the pretty dresses, the galas, etc. Cersei was the one who'immersed herself in the politics and, for better or for worse, that meant that she had a wealth of knowledge about how things worked.
Room For Growth: Sansa Stark
Cersei, however, wouldn't take advice from anyone. She operated on what she thought was right, and was too stubborn to take advice from someone who might actually know better. Sansa, on the other hand, was more likely to listen to her advisors, meaning she might end up with more flexibility and, for that reason, more allies.
Respect: Sansa Stark
Cersei didn't command respect, but she did command fear. Sansa was the one that people may actually have liked had she ruled over the Seven Kingdoms, and presumably liked when she was Queen in the North because she actually was likeable. In of subjects being dedicated when serving their ruler, and in making allies, this gave Sansa a pretty obvious advantage. Even if Cersei wouldn't have liked to think so.
Wealth: Cersei Lannister
In an ideal world, wealth wouldn't matter when it comes to ruling a country — but wealth means resources for the kingdom and although the Starks weren't exactly poor, no one could beat the Lannisters in this regard since they were from a gold-mining country. The Lannisters were so ridiculously rich that Cersei definitely wins out here.
Intimidation: Cersei Lannister
Although Cersei didn't exactly command respect, she did intimidate people which might have put people off committing treason or going to war with the Crownlands. Sansa didn't intimidate people as much because, although they turned out to be wrong, most assumed her to be a young, naive girl.
Fairness: Sansa Stark
Sansa was the fairest of the two. Cersei executed people based on her own personal dislikes, blowing a sept up with wildfire in a frightening likeness to Aerys, who went mad and slaughtered hundreds of his own subjects out of sheer paranoia. Sansa genuinely tried to put her own biases aside (aside from a very satisfying murder of Petyr Baelish) and attempted to be a good queen based on what her people wanted and needed.
Overall: Sansa Stark
Cersei Lannister might have made a good queen, if she had put many of her qualities to good use — if she had used her political knowledge for good, if she had used her wealth on useful resources for Westeros, etc. But ultimately, she was too selfish and impulsive to truly be a good ruler, and Sansa would have been a better one had she sat on the Iron Throne for any length of time.
Thankfully, she ended the series by becoming Queen in the North, so at least one country will benefit from her rule.