Most coasters of all makes and models begin with a turn to the right or left in a sweeping falling motion and then dropping to the required height, there is a reson for this, it's called the coasters footprint and the keeping to a minimum of. (A good example of this can be seen in the signature in the post above mine, thanks jakizle) As such, this style has become... boring I guess, however, some do it better than others. A really bad example of this, has already been mentioned above, Silver Bullet at Knotts. The drop is flatter than Paris Hiltons chest and even slower than her brain! A good example would be Mantis at CP or The Vekoma Corkscrew coasters, where the coaster drops slightly while turning, then proceeds to raise again very slightly before finally dropping, giving you nice airtime as you drop.
For me though, the worst drop I have ever ridden, would be the first drop of Big Loop at Heide Park. Seriously, this is the worst coaster I have ever been on and the "less exciting than an elevator" first drop makes it even worse!
http://www.rcdb.com/ig986.htm?picture=1