$16 (about £10) Bjango, bjango.com/mac/istatmenus
iStat Menus is the most expensive of these apps, but it’s also the most comprehensive. It’s highly configurable and user-friendly, making it easy to set up monitoring for CPU/GPU usage, memory, disks, network traffic, sensors (fan temperatures), and batteries. As a bonus, the app’s Time section provides you with the best world clock on OS X.
When you turn on any individual monitor, it’ll appear in the menu bar, usually displaying a tiny graph, although the visual output can be adjusted in the app’s settings. Click on a menu bar extra and you get a dropdown menu with tons of information; for example, the CPU menu outlines weighty processes, and Battery reveals which apps are using significant energy.
To further check on and troubleshoot your Mac, many of the graphs load a sub-menu when you hover the cursor over them, detailing loads/usages during the previous hour, day and week.