Until recently I thought the most dangerous heresy was the Seeker Sensitive Church (Purpose-Driven, Market-Driven, Felt Needs-Driven, etc) but I am not so sure any longer - I think open theism will have a much greater impact in the long run.

Sola Scriptura! Solus Christus! Sola Gratia! Sola Fide! Soli Deo Gloria!
Total Depravity, Unconditional Election, Limited Atonement, Irresistable Grace, Perseverance of the Saints