Hula is going to be the closest to what is the best. It deals with strictly calendering and mail and forgets all about document collaberation and stuff like that. They figure just stick to the basics. It's based on a code release by Novell for their webmail system.
Strictly speaking it's just webmail right now, they are working on calendering features and it will eventually support a wide range of e-mail clients. Evolution (eventually to be ported to Windows), Outlook, Thunderbird, and they are making a calendering protocol for calendering specific applications.
You see, specific tools for specific purposes, instead of the pre-internet sceme of building a single monolithic application for all your document proccessing, file sharing, user information. Which was what things like Exchange were originally designed for.
Most of what people use them for is calendering and email.. and that's it. The rest is cruft. The majority of it is cruft.
That's what the failure of things like OpenExchange come from. They are trying to meet feature for feature with Exchange and your not going to get community support going for it becuase the vast majority of the community has no use for it. Hula tries to concentrate on simply delivering what people want and what people need and if anybody wants more they have a system of plugins and extensions that people can build on.
Sort of like Firefox browser.
Unfortunately it's not ready for prime time. It's fine for Webmail and personal projects, but I wouldn't want to try to replace Exchange with it, at this point.
If you want something that works right, works right now, will be a feature-full replacement for Exchange, and will run on Linux, check out Novell's Groupwise.
Groupwise is a similar monolythic groupware tool like Exchange and designed around the same time. It runs on a veriaty of OSes and will support Linux clients using Evolution as well as Windows clients running Outlook.
Groupwise. A Suse server running Groupwise is going to cost you, but it's cheaper then a similar setup using Windows. It's not open source. They have a no-cost eval version for playing around with.
(if you dont' need all that and only need email, then you have lots more choices, lots of choices when it comes to anti-spam/anti-virus and all that fun stuff.)