One steel coaster I have ridden that is highly recommended is Phantom's Revenge at KW. CP has one of the best collections of top-notch steel including Millenium Force, Raptor, and now the new Maverick.
Many of the best wooden coasters are at smaller parks. Any wooden coaster at Holiday World or Knoebels comes highly recommended although I haven't made it to either of these parks yet.
As for "bad" coasters, there are some around and many threads here mention them. One of the worst was Psyclone at Siix Flags Magic Mountain (SFMM) but that park mercifully removed it recently.
Nitro at Six Flags Great Adventure
Kraken at Sea World Orlando
Dueling Dragons at Islands of Adventure
Raptor at Cedar Point
Maverick at Cedar Point
Thunderhead at Dollywood
Voyage at Holiday World
Wildcat at Hershey Park
Phoenix at Knoebels
Georgia Cyclone at Six Flags over Georgia