To me green square mode could be a very poor starting point. It depends what you're camera does. My Canon DSLR's used to select green square settings that I was highly unlikely to agree with.
More on that... as I said, it's worth trying and if you're happy with green square mode then you're happy... but... IMVHO there a distinct advantage to understanding the basics of what some would call the exposure triangle and also of understanding the implications of choosing the various settings for the kit you're using and the resultant images. If you do understand what's going on you'd be in a better position to decide if green square mode was worth using.
Green square mode may be a good choice or it may not but I assume that anyone selecting anything as complex and customisable as a DSLR will have at least a passing interest in getting better results than green square mode may provide and for that reason I it seems like a good approach to me to become familiar with the various aperture, ISO, shutter speed and file settings and when selecting them would be most appropriate for you.
"To me green square mode could be a very poor starting point" You have to start somwhere and I think auto is the place IMO. And you do need to learn the exposure triangle. I think Pookeyhead has written on it, but I cant seem to find it as I would have linked for the OP.
Alan, we seem to starting to go round in circles here? I'm not sure I am explaining myself correctly. I use manual mode on my Fuji's and I have previously owned two Nikon D7000. It was a good camera to use in auto, it's what I learned on. This is more modern than the Canon DSLR you used to use? I believe the D7000 to be 5 or 6 years newer technology than your Canon 5D???? and could be used in auto to start learning the camera. It worked for me.
The OP has asked for "best aperture for general shooting". What I originally posted was > "
Shoot in auto and see what the settings are for the photos youre happy with. Use this as a starting point". So I said try auto and if your happy with the photos use these settings as a starting point. If your not happy with the photos still use this as your starting point. To me this means adjust accordingly.
So in auto mode the results are:
ISO 400
SS 1/125
Aperture f5.6
This is your starting point.
Then adjust from there, keeping an eye on what each change does.
ISO 800
SS 1/125
Aperture f5.6
Then you could try
ISO 800
SS 1/160
Aperture f5.6
etc........................................
See where I'm going with this, from the starting point in Auto.
This is how you learn what each adjustment does. I used to start like this, take 5 photos, write down each setting, then adjust and take another 5 photos etc............... keep going until you understand what your adjusting actually changes and your happy with the end result, the photo. If your eyes are good you can see on the camera screen what effect the adjustments make, I usually used to load pictures on computer as I found it easier for me comparing the changes.
I hope that makes sense, we are singing from the same hymn sheet, Im just saying start in auto and take it from there.