The Ring is clearly the most prestigious and difficult to attain. However, going by just alphabet gangs:

WBC (they have the most Ring Champions out of any alphabet gang)

IBF (IBF only get second place because of the WBA's decline)
WBA (The WBA should be second; however, they have severely watered down their titles ever since instituting 3 titles per weight class: super, regular, interim.)

WBO (Although WBO is the lowest of the alphabet gangs it is the quickest rising. In the 1990's HBO would not even mention it, even when it was being defended in their fight. Since the,n the WBO has gained acceptance as a 4th alphabet title.