I honestly feel like the best ending would have been Katniss ending up alone and broken like Haymitch. It is exactly what she deserved.
That being said, I feel like Peeta was the better man. He deserved Katniss more than Gale. Gale was her friend and a good friend. That much is true and I don't really hate Gale (even after book 3), but like Katniss, Gale always put himself and his rage toward the Capitol first. His inability to let go of his hatred, to keep it from morphing him into the type of person whom he was fighting, ultimately cost him any future that he could have with Katniss. Plus, like Katniss, he was too stubborn. He never owned what he did, he never attempted to make amends. Instead he just left. He was just too stubborn to let go of his rage and admit that use of such tactics is inherently wrong.
With Peeta, it was always about Katniss. Everything he did was for Katniss. No matter how badly she treated him, he still put her first. Katniss needed someone like Peeta. Katniss is bleak and pessimistic.....she needs someone calm and optimistic, someone who can get her to let go of her anger, rather than encourage it as Gale would (which is exactly what Peeta did when he attempted to soothe her nightmares).
Plus I like that they wound up together because it kind of plays against type. Peeta is the exact type of literary character who should have died to save Katniss, opening the door for her and Gale. I was waiting for it. Instead she winds up with him.
That being said, I don't like how it happened. There was no realization of love for Peeta. There was no determination that he was perfect for her. Instead, she did exactly what Gale predicted that she would do. She went for the guy who she could not survive without. Katniss had isolated herself from the outside world, she suffered from extreme PTSD. She likely would've wound up like Haymitch (or perhaps worse)....until Peeta shows up. He basically becomes her nurse at which point she just kind of accepts being with him. Peeta loves her so much that it doesn't matter, but in the end, their relationship equates to her continuing to use him.
Instead, I feel like Katniss should've been left to herself. She needed to learn to deal with the trauma that she suffered and continue on rather than simply rely on Peeta or Gale to make it all better.
That being said, in essence, I really like where it all ended, Katniss, Peeta and Haymitch, together again, in District 12. Three broken people who are always the survivors....they have lost everyone else but each other and through that they have essentially become one another's families. I just don't like how Collins reached that point. With the build up that Collins gave, Katniss should've been alone rather than just being dependent on Peeta. I would've preferred a different build up that allowed Katniss to reach the point where the story ended in a more suitable manner. I would've preferred an ending where Katniss learns to survive and cope with what she has experienced and finally realizes her love for Peeta as equals (rather than the boy whom she depends upon or must protect) instead of simply continuing her cycle of using him.
I do feel like Mockingjay shows Collins continued weakness as a writer....she is terrible at doing the falling action. Once the climax hits, it just kinda ends. Katniss kills Coin, accepts responsibility for her role in the suffering and tries to make the world better....five pages later it is over. Same in Catching Fire, Katniss blows up the forcefield....five pages later it is over. There are so many loose ends. What about Plutarch's role in Prim's death? What about Johanna? Why did Gale never say goodbye (and how did that affect Katniss)? What became of Effie? What was the new government like? Did life improve for those in the Districts? These are questions that all should've been answered.
Even her handling of the war seemed far too abrupt. The Capitol was supposed to be an all powerful and all oppressive regime. But basically, Katniss agrees to be the Mockingjay, the Capitol retaliates once with some ineffective bombs that ultimately do nothing, then they fall. Everything just felt too easy. I would've liked more hardship and struggling....a proper war if you will. In Collins' writing, everything always just....falls together it seems.