Originally I wasn't big on the series when it first came out, but a friend convinced me to buy the first one. I did so, and was hooked. I played it constantly, exploring the entire world, killing all knight Templars, planning my strikes, and even collecting all 1000 flags. My personal opinion of the series, its gets better and better.
-Assassin's Creed 1 - Like I mentioned above, this was such a creative game at the time, being able to parkour/climb anywhere and everywhere. Even to today not a lot of games have that freedom in mobility.
-Assassin's Creed 2 - So good it made the first game look like a boring Tech Demo. Everyone loves Ezio's charm, as well as the innovation in gameplay mechanics, and abundance of new things to do and see in a rich city.
-Assassin's Creed 2: Brotherhood - I agree with many that is the crown gem of the series so far. I had my doubts at first, but I saw they took everything we love, made it better, and added even more.
-Assassin's Creed 2: Revelations - This was a good game but admittedly not as good as Brotherhood (that's a hard act to follow!). Less interesting places, new mechanics not exercised properly, and the jump in challenge was strange. It felt like it was more to close up loose ends and prep for the next chapter.
-Assassin's Creed 3 - Mixed opinions on this one. The intro chapters were actually more charismatic and interesting than most of the game. Conner just isn't as likable or interesting, and neither were most of the Desmond missions they had you do. They also changed certain accepted mechanics of the series for no reason, and spiked the difficulty on many segments. The frontier running and naval combat saved this game. If nothing but the naval combat was its own game, I would buy it, it was THAT fun.
And Assassin's Creed 4 looks like its taking the right steps in the right directions. Every gameplay video they put out gets me more excited to play it and pleased with the choices they made. If Ubisoft is pouring as much love into it as Brotherhood, we might just have a new series king.