Definitly try the Rage, they are a good enemy! Most of the TDM-ModPack races are a good shot as well, esp the Xi'Chung!(usually no 1 in the scores)...If your into something different try my Y'Gathian Free States (they use crystal/organic tech) or my Version of the Borg, both in the downloads section.
As always for a challenge start with med tech, high AI bonus and low bonus for AI resources (or med if you are truely in for a challenge)....This gives the AI less 'theory' techs to research and more weapons/armour etc....set the Tech research to low cost and away you go!
Have fun, and remember, be kind to the EEE!
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"The Empress took your name away," said Chance.
Owen smiled coldly. "It wasn't hers to take. I'm a Deathstalker until I die. And we never forget a slight or an enemy." -Owen Deathstalker.