I didn't care for BoF2, the storyline was a turnoff for me.
As for BoF1, it was good too, but I wasn't very happy when my save disappeared after putting several hours into the game.
So, I'll have to say, BoF3 is my favorite. Also the first one I've played from the series, too. It's not too hard either, and I enjoyed the storyline. If I were to recommend any to you, it'd be this one.
Haven't played anything past the 3rd and don't plan on it -- the reviews I've read told me more than enough, though a friend said that the 4th was alright.
When it comes to these things, storyline is a pretty big issue for me. It's what either makes a game great or makes the game a chore to get through -- and no one likes doing chores.