You should use ANSI. Are you typing in just English in the HTML file? Then that's all you need.
See, ANSI is great if the only language in the world is English. Obviously, there's more like Spanish, French, Chinese, Japanese, Russian, etc.. Unicodc figures out how to deal with all these languages. Then you have Unicode encodings, which takes Unicode and makes it usable on computers.
So in Notepad, Unicode stands for the encoding UTF-16 LE, Unicode big endian for UTF-16 BE, and UTF-8. Three different encodings. I would normally recommend you use UTF-8, but you shouldn't in Notepad. That's because it puts something called a Byte Order Mark (BOM). You don't need to worry about the details, but just know that this BOM causes trouble in UTF-8 documents.
I use a better text editor like vim or Notepad++ that allows me to save my documents in UTF-8 _without_ the BOM. You should too. Actually, you are using Notepad to write HTML, right? Wow, that's horrible. Notepad is a terrible text editor.
Just google for plain text editors and download something better. Here's a number of choices: Notepad++, Cream (google "Cream vim"), PsPad, UltraEdit (costs money), Aptana (actually an IDE you might find useful).