There were a lot of rappers and people that Tupac had feuds with, most of which developed after he got shot and spent time in prison.  I think while in prison he realized who were his true friends and who weren't, and realized he didn't have a lot of true friends.  He also realized that things weren't right in the rap game, and I think this is when he developed a plan to set things right.

Other beefs he had, although too small to mention were with Jay-Z, Fugees, Da Brat and he dissed, Junior Mafia, Little Caesar, Little Kim, Little Shawn and others.