I'd like to provide my personal reasoning behind why you might choose any of these editors (at least the ones I'm familiar with). Just saying "use this, use that" is not at all helpful. To large degree, the question is wrong. There's rarely a "best IDE" for a language; rather, there are multiple environments, each suiting particular needs. Here goes:
1) Dreamweaver
Why you would use it: its history as a designer tool makes it much easier for "non-coder" types to start cranking out websites. If you're a solo developer building a lot of "Tom's Corner Store" type of sites, even if they require some CF Coding (mailing list, subscribers, current specials, light content management, etc), its design tools, "template" features, and ease-of-deployment (ftp) make it an attractive choice. It has good-enough code coloring and code completion for the built-in CF tags and functions. It can interrogate user-defined functions in the same page. It has excellent CSS support. You can find a wealth of extensions, too. It's pretty stable and, in my experience, hasn't been very "crashy". It will do a fair amount of code generation for you as well (whether that code is "good" is debatable). All in all Dreamweaver is incredible software for web site designers.
Why you wouldn't use it: It is not free, and it is certainly not a "coder's editor". While it provides for extensions, they're typically interface-focused (javascript validation, etc), unlike say Eclipse plugins, which can run the gamut. For large projects, it simply does not have the code navigation features that many coders come to expect. It's web-focused. So if you're a polyglot, or even just like to dabble in compiled languages (java, etc), then you'll need to keep another editor on hand for those tasks.... you won't be able to do it all in one place. ColdFusion unit testing support is nonexistent in Dreamweaver. There is no step debugging for ColdFusion.
2) CFEclipse plugged into Eclipse.
Why you'd use it: CFEclipse is going on 6 years old now and has matured significantly. It's been quite stable for the last few years and most crashiness has been due to Eclipse itself and not CFEclipse (which was not true in the early days). Recently CFEclipse has seen an infusion of fresh blood and features are being added to make coding in it even more productive. It contains a wealth of keyboard shortcuts, many of the toolbar features people love from ColdFusion Studio days, and Eclipse's in-built code navigation features (namely, Ctrl-Shift-R for finding files quickly).
It has content assist for native CF Tags and functions, and some support for in-page variables, though that's never worked all that well. It does not support in-page functions, nor does it provide native true component insight (i.e. insight into components that you write and use in other code). It will support component insight to some extent with Dictionaries, but even then, it requires a lot of work on the part of the dictionary creator. Most people find dictionaries too much work to maintain, in my experience.
The lastest version of CFEclipse contains the best CFML formatting you'll find.
For me, "method explorer" and "Snip Tree View" -- particularly keyboard shortcuts for inserting snippets -- have been big productivity boosters.
If you work with ColdSpring, ModelGlue, Mach-II, ColdBox, and other frameworks with xml configuration files, CFEclipse's Framework Explorer is brilliant.
Because it's a plugin to Eclipse, you can do everything else you'd want to do in Eclipse. You wanna code java? You can. You want webservice support? you got that. You want to do step debugging, you can do so with the free Adobe-provided extensions for Eclipse.
The large plugin ecosystem is one of the most attractive features of Eclipse, and you shouldn't discount this when deciding on an editor. For example, I would not want to work without Mylyn, which integrates with issue tracking and in my experience has transformed the way I work, much for the better.
Eclipse's version control system support is excellent as well. Subversion is well supported; there's a VSS plugin; and recently a git plugin (if not two) has been accepted into the Eclipse foundation so we'll see native git support very soon (you can get it now with a plugin).
Eclipse's ANT support is excellent.
You can easily plug the MXUnit Eclipse plugin into Eclipse for unit testing your CFML (full disclosure: I contribute to MXUnit).
Finally, I have full confidence that the folks working on CFEclipse -- Denny, Mark, Jim, Peter, et al. -- will continue to work toward keeping CFEclipse as the best open source CFML IDE available. These are some of the brightest minds in the ColdFusion community and are passionate about their mission. If you choose to use CFEclipse, you are not choosing to use an IDE that will be supplanted by ColdFusion Builder. This project is in good hands.
Why you wouldn't use it: it's a code IDE, not a design tool like Dreamweaver. It's not perfect... code assist can be too aggressive in its suggestions. Eclipse itself, especially when you pile it up with all kinds of plugins, can get unstable on lesser machines. Finally, people who don't like the "Project" view of the world often have complaints about it because they're used to working directly with the file system view of the world. Its deployment support is nowhere near as simple as Dreamweaver, though you can find plugins that get close.
3) ColdFusion Builder
Why you'd use it: all of what I said previously about Eclipse itself applies to CFBuilder when used as a plugin to Eclipse. I cannot speak to the Standalone version because as of this writing, it still doesn't support plugins very well. This will most surely be fixed by the time it is released, but I don't want to speculate on what the Standalone may or may not do.
One of CFBuilder's big draws is "Extensions". These are a way to plug in CFML code into your editor. It's hard to describe, so I'd suggest googling for "ColdFusion Builder Extensions", and you'll most likely be amazed. Adobe's Terry Ryan has created "Apptacular" for scaffolding applications from a database, and Brian Rinaldi has a series of posts on building CFBuilder extensions. These are huge and will prove themselves to be a developer's best friend after CFBuilder is released.
CFBuilder's deployment support is, in my opinion, on par with if not superior to Dreamweaver's.
CFBuilder does not require an additional plugin to do step debugging. Just hit the debug button and off you go.
CFBuilder contains true component insight, meaning that it can introspect components you write and provide ctrl-space content assist. It can be wonky, however, and does require some configuration. But please remember that as of now, CFBuilder is still in beta. My best guess is that it'll be at least a few versions until all the kinks are worked out of this feature. Still, it's a big productivity and learning booster to get content assist on your own components.
CFBuilder provides a "Servers" view for stopping/starting your CF Server. It's built on Aptana and so contains the Aptana "tail log" view, which is great for watching log files. Just like CFEclipse, it has a Snip Tree View.
The CFBuilder "vision" is led by Adobe's Adam Lehman. He's passionate about CF and is a force of nature. I have great hopes for CFBuilder because of Adam's leadership.
Why you wouldn't use it:
For one, it won't be free. Noone outside Adobe knows yet how much it will cost, however. "Extensions" and the deployment features alone may be worth the price. Time will tell.
Because it's an Adobe product, I think it's reasonable to assume that releases will come as frequently as most Adobe products, which means... not very often. While CFEclipse deploys rather frequently lately -- and makes available a "nightly" site for the brave -- CFBuilder will most likely not do such daring-do. CFEclipse can afford to make potentially unstable builds available to the public, while it is perhaps not in Adobe's best interests to do so with CFBuilder.
Finally, it's still in Beta and might not be released for some time. If you get it now and start using it, remember that. In my experience, debugging is wonky, content assist sometimes works, sometimes doesn't, and a lot of people have experience crashiness. It's free beta software... you're getting what you pay for. But know that the more you work with this beta release, and particularly if you provide feedback via the public bug database, the better off all of us will be if it provides a best of breed editor for CFML.
Personally:
At home, when I do "designer" work, I use Dreamweaver when I feel that its Templates will help me build a site as quickly as possible. For existing side projects which require maintenance coding and easy deployments, I use ColdFusion builder.
At work, where I do almost no design work, CFEclipse has been my IDE since 2006. I've begun using ColdFusion builder a lot, though currently I split my time between CFBuilder and CFEclipse. One reason is that as of this writing, CFEclipse is more stable (i.e. it doesn't crash and I don't lose work). I fully expect stability problems to be mitigated by the time CFBuilder costs money.
Both CFBuilder and CFEclipse have public bug databases. CFEclipse has a well-attended public mailing list, and if you have questions, you'll get answers quickly. I cannot yet speak to the speed with which CFBuilder questions are answered.
Finally, for "coders", it's my experience that once you invest the time in learning the tools and shortcuts, Eclipse provides superior productivity compared with designer tools like Dreamweaver. For cranking out a designed site, a designer tool like Dreamweaver confers significant advantages.
The answer to the best ColdFusion IDE isn't an answer, but a question: "What are you trying to do with ColdFusion?" The answer to that question will lead you to an IDE that suits your needs for a particular project. Different circumstances or projects may lead you to a different tool which better suits your needs.
