Listen to each song and pick your winners based on things like cowbell usage (unique? loud? continuous?), song's success, song's age, how much you like it (try not to get carried away with that one), etc.
The 1976 song that Will Ferrell and Christopher Walken chose to pick on, likely resulting in a resurgence of popularity for the song (which is already the group's most famous and reached the top 10 on the charts). The cowbell is there, but not to the extent of others in the competition.