i currently have a very comfortable and great performing setup but i'd have to say the one out of it that i couldn't easily replace with some other similar piece of hardware is my Logitech G600 mouse.
for some reason the description states it's an "MMO" gaming mouse, but i've never been into them at all and it is perfect for every game.
the size, the weight, the layout, and the available options are all perfect for me for browsing, in-OS tasks, or gaming.
it's 3rd click, or 2nd right-click, is the only time i've seen this feature on a mouse that gives the ring finger a dedicated resting place that is also a button.
on the fly DPI adjustments and a great sensor make it perfectly smooth and accurate.
the amount of buttons on top & side make every task and every game much easier. RCtrl, Home, End, Up, Down, Left, Right, Backspace, Enter, Delete, +, -, PgUp, PgDown programmed in and all at my left hand finger tips.
Plus the right\left tilting scroll wheel for the Mouse3 & Mouse4 buttons that most games and Windows has made use of for years now.
my next favorite would have to be my Alienware ultra-wide 1440p 1900R display. Alienware products never appealed to me in the past due to their mostly ugly appearances and over the top design but the
AW3420DW is beautiful both aesthetically and with it's display capabilities.