For me it's the 3er iDrive.
3er: It looks the best to me. I have sat in all cars with the iDrive option, except the new X5, and i have to say, that in the 3er, the screen fits the interior perfectly, and it makes a great combination.
5er: A dad's friend has a fully optioned 5er and i have had many rides in it. He is a BMW fan, too, even though he is more than 60. Anyway, the iDrive screen looks good, but kind of big, compared to the dashboard and the whole look of the interior of the car. I would like to see the screen fit a little better. Other than that, i like it. I don't have any problem with the buttons.
6er: Here, in this car, the iDrive screen looks completely out of place to me. It's like it shouldn't be there, but someone found a way to fit it there. I don't know why i don't like it, but it may be bacause of the CF trim, which i hate, and which does not fit with such screens and all that technology. I think it would look better with a wood trim. After all, no race car (CF trim) has such a screen in the interior.
7er: My second favourite. It looks like the whole interior has been designed based on the screen which looks like the centre of the car, the centre of the world. I like it very much, but am not very familiar with the fact that the screen is the first thing you see when you enter the car.
Χ5: Great interior, the screen fit's perfecty, but the whole look of it is not my cup of tea. I just need some time, and i will become a fan of it.