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Also the ability to highly customise META tags / tiles for SEO is a must.
At the same time the CMS should be easy and quick for building the site, and not too difficult for the eventual content managers to use after go-live.
Which CMS would you recommend for such a project?
Thanks
If you find some packages that meet your requirements go try some of them out for yourself at the opensourceCMS site. They have a bunch of systems pre-installed that you can test out yourself. It's a great time saver.
very complex design
Generally, most CMS are fully skinnable, so the design is not usually limited by the CMS, but by the mind and skill of the designer.
fully adaptable
If by that you mean "any design I want", see above. If you mean that it can adapt from page to page so that you don't have one sitewide template, some CMS are better than others. I personally think a sitewide template is best, but some people like one color header for the forums, one of the blog or some such thing.
What's good for one developer might be too complex for another.
I couldn't agree more. Take the name of almost any CMS and try these two searches:
"[CMS name] sucks"
"[CMS name] rocks"
If you find a CMS that doesn't return results for both of those searches, it means that nobody except the developers themselves are actually using it.