Co-op would be pretty fun. It'd be alot easier to take down groups of Raiders and Enclave. You could have roles. One guy could be up front with big guns, and the other(s) could be back sniping or lobbing grenades. I've learned that tactic after several raider battles.
On the other hand, I think that a MMO could be fun. The thing about massively multiplayer games is that it's a living, breathing world. Instead of battling the same enemies in a static world, you could form factions, control territories. You would be forced to think and change your strategies instead of applying generic tactics based off of what enemy you face. The gameplay would be fueled by the other players. Your weapons, clothing, items, locales, friends, and enemies would be affected by your experiences with other people.