I used to be big on getting 100% on achievements, but these days I try not to spend an insane amount of time on a game just for achievements. I figure I'd rather spend my valuable time playing a new game.
Anyway, the games I have gotten 100% achievements on:
-Fable (Fairly easy except for the online achievements which I pretty much boosted with online friends)
-Elder Scrolls IV: Oblivion (This was easy, just takes time, just do every quest and you will get it)
-Marvel: Ultimate Alliance 2 (All easy except for one challenge which took me WEEKS to finally beat. Time I will never get back)
-Assassin's Creed II (Easy, just took time)
I'm also pretty close on Skyrim, but I'm not rushing to get the achievements since I want to take my time on finishing the game to draw out the experience.
One thing I hate is when DLC is released with new achievements that ruins your 100% status. I very rarely purchase DLC
Well, I'm sure some of you have heard of the game Avatar: The Last Airbender. All four achievements are for getting combos, and you can get them right at the beginning of the game in the tutorial. This is exactly what I did, and it was the first game I'd ever gotten all achievements on.
I remember seeing videos of this. I was tempted to buy the game just to get 100% achievements on it.