The 1918-D is a common date in the Lincoln Cent series, though its mintage is well below that of the ultra-common dates such as the 1918-P. Collectors may choose between Brown, Red-Brown, and Red examples, of which there is a sufficient supply to meet most demand. The key aspect of this date is the strike quality. At Denver, dies were used even after they became worn, thus many 1918-D Cents have mushy details or surface granularity. When seeking out a 1918-D Cent, look for examples with a crisp strike and hard, lustrous surfaces. As with all Lincoln Cents (and copper coins in general), shy away from overly spotted examples or those with unattractive toning.
After more than two decades of certification events, the most common grade for a 1918-D Cent in Red-Brown is MS64. Even MS65RB examples can be found without too much trouble, but none have been graded and finer by PCGS.