thats not an easy question
I got all these games and all of them are top of the line
Im gonna say
Fallout 3 > Mass Effect 2 > Mass Effect 3 > Fallout New Vegas > Mass Effect 1
Mass Effect 1 had a great story but the gameplay did not age well at all, but if you make it through it, its a very rewarding experience especially when you get to Mass Effect 2. The first thing you notice is gameplay, its so damn good. Plus all the choices you made carry over, good and bad.Mass Effect is one story you make a character and stick it out through the whole series. Thats what makes it dope, this is your story however you want it to go. The decisions you make carry over to the next game and all of them carry over to the 3rd. For the best experience you have to play them in order.
Fallout on the other hand you dont need to play one to know whats going on the other one but I recommend playing fallout 3 over new vegas.. One important thing to know about Fallout, fukk the main story, explore the world. There is so much to see in both of these games
The real question is do you want to Explore (Fallout), or do you want a great story (Mass Effect)
Skyrim may be better than both..... but since they are still releasing dlc you could wait on it and get it for cheap with all the dlc cause they are gonna release a game of the year edition sometime next year