It really boils down to your goals and skill-set. Generally for most seasoned web designers it breaks down in order:
A. Static Design Tools:
1. Photoshop
2. Fireworks
3. Keynotes (Yes really)
4. Illustrator
5. InDesign
6. Pixelmator
B. Dynamic Design Tools
1. Coda 2 (has builtin preview browser)
2. Sketching on paper + hand coding CSS (think designing in browser) + your choice of editor without a preview. (Sublime Text popular pure editor, and of course TextMate) + Browser of choice, most go Webkit based from an OS X perspective.
3. Espresso 2 (has builtin preview browser)
4. BBEdit (has builtin preview browser)
Of course this is not limited to the options above, I'm sure there will be a few special snowflake options but those would be more developer or amateur options. Again if you're going for the most popular and supported tools amongst web designers, there you have it.