Difficult question to answer. It all depends on your defensive players strengths and skills. All of them are effective with the right group of guys.
Personally, I believe that a hybrid defensive philosophy, where they play a little of everything, is best. Rex Ryan is fantastic in Baltimore at doing this with his D. I was watching a Ravens game where they were playing the Broncos... it was 1st and goal for the Broncos, on the 4 yd line. The Ravens initially set up in a goal line defense (8 men in the box), but before the snap there was an audible that dropped every single player, but one, back into coverage. One DT on the line on the goal. Obviously they sold out on the pbutt. Well, it paid off big time! Cutler dropped back and looked like a pop-warner QB. He ended up trying to run it and got dropped in the flat, no gain. The more looks you can throw at the opposing QB the better, I say.
I say 3-4. Brings out the most talented and athletic players, while the best minds in football get to work with blitzes and so on.
Cover 2 is pretty fun to watch, alot of big hits and picks due to corners and safties being able to gamble more.
I agree. Being a Falcons fan, and having to watch the Bucs twice a year has given me an appreciation for the Cover 2/Tampa 2. The safeties in this scheme are more tweener/corners, and the CB's are used a lot against the run. But I love to watch the linebackers fly around the most, ala, Derrick Brooks. Doh!!!
I think they have all proven effective and all proven to be the best as long as the proper personnel is in place.