I am a great fan of Logitech kit as it is quality and they have fantastic customer service. I got in touch with them when I had an intermittent problem with my G400, they sent a prepaid label and then replaced the mouse.
Back to your question.
As I am not a great lover of loads of buttons I currently have a Logitech M518 which is my favourite, then the updated version the G400 and lastly a Mionix Saiph 3200. All these are comfortable to use for long periods. Which is important as I have bought mice that I knew within a few minutes that I would not be keeping them.