Draft Express has been very good.
However, last year they flubbed (like every other mock draft).
To be honest, there is not a premier person at NBA mock drafts.
If I had to separate some from the field it would be DraftExpress and Sam Amick (SI).
Sam Amick had a great mock draft in 2011 (Mock Draft 4.0), until he started listening to guys like Chad Ford and Joe Kotoch which forced him to change his. In Amick's 4.0 draft he correctly picked 7 of the first 9 draft picks. So I guess you could look at Amick's mock draft from two weeks ago and go off that lol. I actually don't know if Amick has done a mock draft this year though.
I'll also point out that DraftExpress is the only draft that had us selecting Kemba Walker in the 2011 Draft.