I was tagged for this by my favorite literate primate. How could I let her down?
n.b. “Perfect,” as I will use it below, doesn’t always mean perfect but rather perfect in its context or else merely “really terrific.” Especially when used of actual, historical people. As we all know, perfection isn’t perfect, right?
The Perfect Genre
“Pick a book that perfectly represents its genre.”

The Rise and Fall of Ben Gizzard is the perfect Western.
“Ben Gizzard would die on the day he saw a white mountain upside down and a black bird talked to him, but not before. An old Indian he cheated out of some furs told him this.
“This was good news to a man as mean and crafty as Ben Gizzard. He settled in treeless, birdless Depression Gulch and cheated, robbed, and killed his way to riches. How his life seemed charmed in that place where there were neither mountains nor birds!
“But one day a young artist arrived in town with a large black bird sitting on his shoulder. Oh, Ben Gizzard!
“Our slithering villain comes to his end when he least expects it, and the world is a better place without him, and a better place for the telling of his story, which is both funny and awesome.”
The Perfect Setting
“Pick a book that takes place in a perfect place.”

Gosh, there are so many books that I love for their setting. There is Pearl Buck’s Pavilion of Women (an aristocratic household in pre-Mao China), Susan Cooper’s The Grey King (Wales, with magic), and Alexander McCall Smith’s No. 1 Ladies’ Detective Agency series (modern-day Botswana).
But my top pick would have to be Treasures of the Snow by Patricia St. John. It takes place in a beautiful, sensuously described version of Switzerland (example: the children picking fresh strawberries and eating them with cream for lunch) and, of course, the fact that everything is so mountainous is crucial to the plot.
The Perfect Main Character
“Pick the perfect main character.”
I’m gonna have to go with Bilbo Baggins here. His combination of humility and spunk cannot be beat.
The Perfect Best Friend
“Loyal and supportive, pick a character that you think is the best friend ever.”
Sam Gamgee would be an obvious choice, but there are class issues there, so instead, let me name Mr. Tumnus from The Lion, the Witch, and the Wardrobe.
Reason: Tumnus just met Lucy an hour or two ago and she has not done anything special for him … and yet he’s ready to put his own life on the line to protect her from the Witch’s secret police. “Greater love has no one than this, that he lay down his life for his friend.”
The Perfect Love Interest
“Pick a character you think would be an amazing romantic partner.”

Charles Ingalls from the Little House series.
On the down side, he will drag you and your children across the American frontier, where you will almost lose your lives in every. single. chapter.
But on the plus side! The man can build a livable house, single-handed, in a few weeks. He can dig a well, provide food, and make friends with the Indians. He’s never met a stranger. He is gentle and kind with Caroline and his daughters, and he is unflaggingly cheerful, even when starving. (Read The Long Winter and watch him effusively praise the dehydrated cod gravy that Caroline breaks out to put some variety in the family’s totally inadequate diet.) And he can fiddle, sing, and dance! What more could you ask for?
This guy is a ball of energy and good cheer. There would be no better person to have by your side in the hairy situations that he will surely get you into.
The Perfect Villain
“Pick a character with the most sinister mind.”
“Amazing Amy” from Gone Girl. I don’t think it will be too much of a spoiler, by now, to tell you that she fakes her own death and frames her husband for it, then fantasizes about “him getting butt-raped in jail.” It’s never clear what Nick did to deserve such a fate, other than fail to think she is sufficiently amazing. And that trick she pulls at the end of the book … well …!
The Perfect Family
“Pick a perfect bookish family.”

Since I was a little hard on the Dutch on Friday, let me rehabilitate them a bit. My “perfect bookish family” is a Dutch family, and they actually lived: the ten Boom family of Haarlem, circa 1935.
Corrie ten Boom’s book The Hiding Place describes how the family took in Jews during the occupation until they were turned in and went to concentration camps themselves. Even though the book is about the Holocaust, it is heartwarming and includes many laugh-out-loud moments. The heartwarming part is Corrie’s description of the family’s life in their tiny, ramshackle house/watch shop. The laughing out loud comes mostly from the personality of her father, Casper ten Boom, a true character and an amazing man of God whom I look forward to meeting some day. Corrie’s mom was also amazing, and I think it was her warm personality (and Casper’s) that offset the natural sternness of the Dutch of that time, making the ten Booms … the perfect bookish family.
The Perfect Animal or Pet
“Pick a pet or fantastic animal that you need to see on a book.”

The humble chicken.
There are a few books with a chicken protagonist. The Book of the Dun Cow, by Walter Wangerin, Jr., is one. There is also a Russian fairy tale where a rooster saves two poor children:
“I, the cock, have a crimson comb / And the wicked czar has nothing like it! / He took away their fortune / from two poor little orphans / And he dines in style / while they go hungry!”
The Perfect Plot Twist
“Pick a book with the best plot twist.”

Stern Men by Elizabeth Gilbert.
(Yes … it’s about lobstermen.)
Though it takes place on a tiny island in Maine, this book features a Jane Austen-worthy plot twist near the end.
The Perfect Trope
“Pick a trope that you would add to your own book without thinking.”

Spiritual transformation of a main character.
Not only would I add this without thinking, I actually wouldn’t think of writing a novel where it did not happen. To me, spiritual transformation is a critical part of a novel.
(Not that I don’t enjoy books where this trope doesn’t take place. Mystery series, particularly, do well if the detective MCs are fairly static.)
There are a few novels where the transformation is almost the only thing that happens. I give you The End of the Affair by Graham Greene, and all the Church of England novels by Susan Howatch. But much more commonly, the MC’s spiritual transformation happens as a result of a lot of other action in the plot, as in The Hobbit (fantasy), Identity Man by Andrew Klavan (crime thriller), and many, many others.
The Perfect Cover
“Pick the cover that you would easily put on your own book.”

My book is not St. George and the Dragon, but this is the artist I would have wanted to do my covers: Trina Schart Hyman. She’s gone now, but her art lives on. I have been trying my whole life to draw and paint like she did. I’ll bet she would have made the ruined Tower of Babel look amazing.
The Perfect Ending
“Pick a book that has the perfect ending.”

A Christmas Carol.
It’s the ultimate happy ending. We feel Scrooge’s childlike joy when he realizes he is being given another chance at life. Also, Tiny Tim is not dead after all and Scrooge has a chance to save him. “The spirits did it all in one night!” There is a strong sense of death and re-birth, not just of Scrooge but of his entire world.
I realize that everyone knows how the book ends and you might think of it as a cliche at this point, but really, if you read the entire book, hanging in there through Scrooge’s sad childhood, slow hardening, the horrific descriptions of poverty, etc., and then you get to the end and it doesn’t move you, well, I don’t know what to do for you, really.