The Sanctuary of Stone and Fire is by far my favourite level in all Rayman games. It is, in my opinion, the one that reflects the most the magic universe of Rayman 2, with an ancient temple in the depths of the earth, in the middle of lava rivers and bottomless pits. Whenever I play it, I can't help wondering what's behind the walls or at the end of a lava flow, who built the sanctuary and why, etc. This is certainly the longest and probably the hardest level in the game, and I can understand why it's frustrating at first try (I remember spending a whole day on it when I was a kid, and swearing I would never play it again), but once you master it, it's a real pleasure to go through this magic level. From a gamer's point of view, the level is far from linear, as it's a real maze, and there are various gameplays, such as plum riding, shell riding and sliding. And of course, the music is great, gets more and more mysterious as you go deeper in the sanctuary, ending with a real masterpiece when you wake up Umber.
As concerns the place where you have to ride the plum with a magic sphere, it's extremely easy, once you've understood that the game does it all for you. Carry the sphere, fall onto the plum, turn backwards, throw the plum in the air and shoot an energy ball, and the plum will just follow you.
In conclusion, I really love this level, and, to answer your second question, I don't think there are any levels that I hate. I find the Top of the World a bit boring, but well, playing Rayman 2 is always a pleasure
