What is the most important program on your computer other than internet browser?

Kaspersky and Malwarebytes I expect. Both are hopefully helping me secure my computer system as nowadays even geeks can be caught by very clever mischief makers. I've used Kaspersky successfully for many years, but am getting worried the paid version is getting too bulky these days. I use the Malwarebytes free version which means it's not online - I feel I only need to scan my computer once in a while. For someone who does more risky searches however the Malwarebytes paid version would be better to use. I also use Notepad a lot, both the regular Notepad for making notes in, and Notepad++ for programming.