美丽：泛指从形到神给人以美感，适用范围较广，可以形容人，习惯上指女子或者具体、抽象的事物； 漂亮：着重指外表好看，使用范围较窄，常形容具体事物，不形容抽象事物，但形容人时不限男女。 例：她不仅长得漂亮，而且有颗美丽的心灵。

Michael answered your question well. I'll just answer the 2nd part of your question. If you're expressing "You have the most beautiful eyes", I would say: 你的眼睛最美丽/漂亮。 If you're expression "Your eyes are the most beautiful", I would still say: 你的眼睛最美丽/漂亮。 "你的眼睛是最美丽的" is grammatically correct. But... it's like saying I want you to know that your eyes are the most beautiful features on you. 我的房间是红色的。(My room is red.) Here, I'm emphasizing the fact that my room is of the color red. 我的房间红色的. (My room is red.) Here, I'm just saying that my room is red. In this example, there isn't that much of a difference, really. In day-to-day conversations, you can get away using either one. But let's not forget also, that what we say also depends what we were asked and what we were trying to express/relay to the other person..

是 有时候用有时候不用。 比如：你的眼睛是最美丽的。这个时候用。 2. 你的眼睛最美丽。这个时候 是 和 的 都不用。 美丽 same as beautiful 漂亮 same as good looking