I'm currently using Mission Center, which is also really good. One of these should replace GNOME's system monitor tho because it honestly kind of sucks.
Gnome
The GNOME Project is a free and open source desktop and computing platform for open platforms like Linux that strives to be an easy and elegant way to use your computer. GNOME software is developed openly and ethically by both individual contributors and corporate partners, and is distributed under the GNU General Public License.
Yeah, I find it really nice to use but I experience a lag spike when I open Mission Center.. so that's a bit annoying.
Should probably report that to the dev
I'd call gnome system monitor good enough but out of place from a layout perspective. Its looked the same for as long as I can remember
I also personally don't like the mission center layout as much. It feels like they copied Windows for no good reason. However, that's just a personal preference
They definitely copied Windows but Windows' Task Manager is honestly one of the few things I actually like about Windows. My issues with Gnome System Monitor is that it's missing important information, information isn't easy to read, I don't think there's a way to stop an application and it doesn't look good.
It probably will be rewritten at some point. For now we have the resources app
Resources and Mission Centre are both really good. I never know which to use.