The_Universe said:
Why chaositech? What's the quality that sets it apart from the other releases of 2004?
I've got to say it's more the flavor of the Chaositech than the rules. The rules have some problems, and don't do a whole lot innovative. (Edit: The do do some thing people may have wanted, but use existing systems like "mutant" templates. So if that's worthy of a nomination, there you go.)
The Universe - are you specifically looking for variant rules, or do you want bits like creatures, templates, classes, and the like?
Some possible inclusions:
If you include specific examples, not just variant systems, there is a lot of fresh creature fare in Denizens of Avadnu and Advanced Bestiary (I was particularly intrigued by the Four Horsemen template, among others)
Ronin Art's Visions 2 has some nifty rules for handling ghostly manifestations.
Unearthed Arcana has had a few entries mentioned. My favorite bits from it are action points (a bit different than Eberron and D20 Modern) and LA reduction. Racial level are also sort of neat, but I'm not personally using them right now.
Though it was originally in a book more than a year old, Hyperconscious' mindscapes combat system is an excellent system for handling psioic combat and making psionics more distinct from magic. It also has some excellent and flavorful character choices for psions.
Hamunaptra, in addition to being an excellent flavor supplement, has a nice non-magic spin on the ranger, and a less pathetically weak spin on the sorcerer.
Mongoose's
Book of the Planes has some nifty mechanics for planes too, including nicely encapsulated organizations with feats and brief classes, as well as a neat system for describing portals and planes (though I must admit I like those in Portals & Planes a bit better).
Malhavoc's
Book of Iron Might has a nifty maneuver system that has lots of swashbucklery potential and some interesting feat variants. Ironborn are nifty too, but I don't know if I'd have them make the cut for a "best" collection.
Is Mongoose's
Book of Dragons less than a year old? (It seems not).
Several other things I like have already been mentioned (elements of magic, etc.)