which is the best stackoverflow engine out there - any idea?

[dilbert2010]

dear community,

i plan to run a q and answer - engine that is similar to stack-engine

question: which one is the best one

cf : http://meta.stackexchange.com/questions/16054/is-the-stack-exchange-engine-available

Is the Stack Exchange engine available? - Meta Stack Exchange

note: a engine in php would be nice - since this is running on my server
i prefer one that is running on many many sites and that has got many features..

any guess?