One of the hardest things you can do to a franchise is to drop the ball of the series. While it could be easy to change the location of a game, and still have that same feel, but it's another thing entirely to change up the game itself. Sequels should improve upon on what is there, and expand the world by changing up the formula. Sometimes the sequel can be so good, that it surpasses the original in every way, sometimes to the point where the original can no longer be enjoyed.
ASSASSIN'S CREED 2
When the first Assassin's Creed game arrived in 2007, it had a polarizing effect on gamers. Some liked the game play, while others hated the repetitive style of the missions. Two years later Assassin's Creed 2 was released. The sequel improved upon the original while expanding the gameplay making it feel fresh. It had more missions, more areas, more characters, and more story. When I finished Assassin's Creed 2 I when back to the original game, and I found myself not liking it as much as I did when I first played it in 2007. Assassin's Creed now feels boring to me. With Assassin's Creed III coming out, it's hard to tell of Ubisoft can do it again.
In Portal 2, Chell has woken with a help of a sphere named Wheatley, soon after Glados is waken as well and sends you down to the deepest, darkest part of Aperture Science. This is where the game starts twisting your mind. With gels that can do different abilities, you need quick fingers and a quick mind to get through the challenging puzzles alive. The story is expanded, and the game is much funnier. It's hard not to be mad at something that constantly makes you laugh as much as Portal 2 does. It's longer, smarter, hysterical, and it exceeds the original in every way.
UNCHARTED 2: AMONG THIEVES
Uncharted: Drake's Fortune is one of the early flag ship titles of the PlayStation 3. While the series star, Nathan Drake, had a lot of character development in Fortune, he didn't truly come to his own until Uncharted 2: Among Thieves. Everything about this game is vastly improved; the gameplay, the characters, the story, and the set pieces. No matter how many times I play the collapsing building or the train levels, I always have a smile on my face. To help Nathan on his quest he has a few more friends to help, like series newcomer Chloe Frazier and fan favorite Elena Fisher. While Uncharted 3: Drake's Deception is a great game it doesn't match the magic Among Thieves has.
MASS EFFECT 2
The original Mass Effect was a masterpiece of great storytelling. That was until Mass Effect 2. Mass Effect 2 takes everything you love about the original and perfected it; The story, characters, gameplay, and universe. I don't consider the Mass Effect series as a pure RPG. It is, however, one of the best shooters around. This is one of the few games that felt that each mission has weight. Main missions or not, each mission is worth playing. The Mass Effect Universe actually feels more alive than ever before. It's game world where I would actually like to visit.
I didn't care much about the original Borderlands; It felt monotone, it was empty, and it felt that I was doing the same thing again. When I got to play Borderlands 2, it was nothing like the original; It was exciting, it was alive, and nearly everything I did felt new. Everything about this game is better. This is what I wanted Borderlands to be. Usually with RPGs, I play with one class and one class only. With Borderlands 2, I would have gladly play with the other classes. You are constantly upgrading either by leveling up, badass rank, or weapons. There is always something new to do in Borderlands 2. This is easily my top 3 games of the year.