Whilst agreeing whole-heartedly with Medium Rob's selection of fictional journalists, I'd also offer the following:
Nick Mullen (Gabriel Byrne) - Defence of the Realm: righteously pissed off.
(and honourable mention for wondeful Denholm Elliot as Vernon Bayliss in the same film)
Amy Archer (Jennier Jason Leigh) - Hudsucker Proxy: "I'll stake my Pulitzer Prize on it!!"
And whilst obviously the model for wicked journalists goes back to Ace in the Hole, Danny DeVito (Sid Hutchings) in LA Confidential certainly deservces acknowledgement for being such a recklessly enjoyable character!
UPDATED: well, at least I got Rob's link in the post now!