Which is the best alternative for Dreamweaver in Mac?
Adobe Dreamweaver (for Mac) — $399
As Andrew Larsson said, there is Mac version for latest Adobe Dreamweaver (url is the same as for Windows version).

If it's a tool that you like and already own, but it's not supported by Mac, then you may try to make it work under OSX using Parallels or wine. Some people made different Dreamweaver versions to work on wine (on linux), so you may try to install wine on Mac and then install Dreamweaver in this case.



Fraise

Easiest and most user friendly editor I've found so far is Fraise. It is small and nice, there's no 'workspaces' like in Eclipse and key-combinations like in Eclipse and Vim. It just stays out of your way and allows to edit HTML (there is preview window for it), CSS, and a lot of other file formats. I haven't seen yet any possibility of code-completion, but is it a big deal?



Ironically you may need to build it from source (but it's very and fast if you know how to use git and Xcode)



Quanta Plus
They say that it supports both code editing and WYSIWYG and it definitely works on OSX so you may like it. I've never tried it though. 





There are also at least 2 tools based on Eclipse that you can use for web development:

Eclipse IDE for JavaScript Web Developers

It incluses tools for JavaScript developers creating Web applications, including a JavaScript IDE, tools for JavaScript, HTML, CSS, and XML.

As alternative you can install any other eclipse but make sure that it contains Web Tools or install them in eclipse of your choice:





Aptana Studio
It's also based on Eclipse. Besides HTML, CSS and JavaScript it supports server-site scripting languages (Ruby, Python, PHP). You can preview web pages you edit in built-in browser, but I don't know if it is possible to edit webpages visually in Elipse based IDEs.