Which would be your favourite type of backline and how would you play it?

It depends on your squad. If you have wingers who can play both sides of the game well and are fast, you can try having 3 central defenders. Then your formation can return to 4 or 5 men defense in the game when needed.

But to be realistis I would go for 4. Every defender out there knows to play a variation of 443 as the back of their hands. I would not mess with my backline and search innovation elsewhere.

Zola is right,it all depends on the squad, but if you are asking me which is my favorite my answer is a back 3. The way I would play a back 3 is simple. They don't attack and stay defensive all the time. Also if I were to play with a back 3 I would get outside midfielders, or wing-backs that can attack and support the back 3.

There's a reason why most of the teams play with 4 defenders. I too would go for it.

What bothers me is the belief that taking a man out of your backline and introducing a player in midfield or attack makes your team more attacking. This is so wrong. It is not the formation that makes a team attacking. It is the mentality and skills of the squad...

3 men defense is no silver bullet and a manager need not try it unless there's a very good reason.

You are right captairgerard it all depends on the mentality and skills of the squad, but a back 4 is no silver bullet either.

Here's a thought.
If you use 3 at the back, you would probably get them to stay deep and concentrate only on defending. With a back 4 (if you have forward thinking full backs), you run the risk of being left with a back 2 if a move breaks down and the opposition counters quickly.
Isn't there an argument then that a back 3 could be seen as more defensive?

Yes, there can be an argument that a back 3 could be seen as more defensive, but if look at most teams that play with a back 4 the full-backs do attack, but not as much to cause problems for the defense.