Final answer:
A desktop computer is the most powerful and easiest to upgrade among the options, while being less portable than laptops, tablets, and smartphones.
Step-by-step explanation:
The most powerful and easiest to upgrade device among the given options is a desktop computer. Desktops provide the space and compatibility necessary for upgrading components like the CPU, GPU, RAM, and storage. Laptops, tablets, and smartphones are designed to be portable, with laptops being somewhat upgradable, but tablets and smartphones are usually very limited in this aspect due to their compact and integrated designs. While desktops are less portable, their power and upgradeability make them a favorite in scenarios where performance and longevity are key priorities.