Most comfortable mouse is...a trackball!
Personally, I agree.
But the OP asked for a mouse (as archaic and straining of a device it is over time). While I have nothing to say on current marketed devices, I will give fundamentals on what I will look for (if I do) in a lift and drag mouse.
Buttons and the number of them are nice and all for saving keyboard input and shortcuts, this is the LAST thing I will consider.
Weight, first off, is the biggest concern. If it is heavy, lifting the damn thing will be tiring and sore for my wrist and arms. Consider the glass of water problem. Not heavy initially, but hold it for an hour without setting it down. While mouse usage may not come one to one with that comparison, it holds true.
Second and third being shape and size of how it conforms to my hand so I do not feel cramped gripping a small object. If it is too large, I might strain finger reach for buttons.
So in essence, it is trying on a pair of shoes. Best way to have a wide array of selection and to try them without any hassle, is to find retailers online that have a hassle free return policy. Trying it in store may help, but it does not allow you to fully assess the mouse in your workflow for comfort.
The fourth thing alongside weight, shape and size, is the drivers and software. Get something that has solid drivers for your operating system. All other keying software, in my opinion, are a bonus if you can work them to your workspace and flow.
And also of my own personal criteria, the mouse would be wireless. I dislike the reliability of bluetooth at the moment, having owned a no name travel mouse that is uncomfortably tiny, and the Logitech diNovo combo. The dongle would have to be small, and be able to broadcast (even if it meant an extension cable from the back of a rear USB port).
Good luck in assessing your situation.