You might have noticed that I love Davis Cup. I can't help it, my sense of nationalism is stronger than me. There's nothing like sitting with a crowd that really cares about the outcome of every point, to cheer together, to feel you really motivate your players and help them win.
But it's more than that - I love watching World Group matches, no matter which countries they involve. You just feel that the players do everything with a completely different motivation when they're on court, and I really like to see that. Just look at what winning DC did to Serbian players, or better yet - go and watch "Happy Dancing Spaniards" :)
Honorary mention: World Tour Finals.