I use gmail, yahoo, outlookl. I stopped using gmail as much when I was looking to buy a vehicle and started sending a few emails out. I noticed the advertisements started to change based on this. So did a google email test where I sent emails and put information in them about specific items in the text area, and sure enough it looks as though google actually reads your emails to cater their advertising to you. I wasn't to comfortable with that, so now not using it very often.
Also can't stand the default conversation view in gmail. Not sure why so many email services have started to follow this. Maybe I am one of the few who like the old way of sorting emails. give me by date in the order I receive and I am happy. At least you can adjust.
In reality outlook is probably the best, but for some reason don't use it often.
FYI..Isn't google the great "big data" satan? I am surprised it is number one...Kind of.