System Load Indicator is the default system indicator on Ubuntu. I like to keep it running at all times in the top panel, showing me graphs of CPU, memory, network and harddisk usage. On clicking it, the numerical values of these entities is displayed. The System Monitor app can also be started by clicking it.
$ sudo apt install indicator-multiload
The only feature I wish it had was to show usage for individual cores of the CPU. The cumulative usage is pretty useless since most applications are single-threaded on a desktop workstation.
Tried with: System Load Indicator 0.3 and Ubuntu 14.04