You have essentially a couple of different type of options:
tools that embed themselves in the menubar e.g. iStat Menus, MenuMeters etc. These give you the best overview, at-a-glance in my humble opinion. Often clicking on a small icon gives you more detailed info.
tools that are stand-alone or integrate themselves into the notification center. iStat Pro, Monity etc. I find these a bit frustrating to use since I need to actively look for the stats I am interested in, rather than just checking the menubar. But the advantage is that they have more real estate to show graphs, give stats etc
command-line based utilities. These are essentially behind all other tools. If you are mostly working in a terminal then you cant really beat
htop in my opinion. it's not installed by default as far as I know but should be easy enough to get via homebrew.
You can also run
fstab and similar command-line tools from tools like GeekTool (mentioned before), or my current favorite, Ubersicht
They allow you to have custom widgets, for example integrated top instance running in the background. My problem here is that my desktop is overcrowded, both in real life and virtually on my computer :) So I figured I needed to do a swipe gesture to be able to see anything
I have tried several different solutions, but in the end I came back to iStat Menus, since no other solution gave me a good enough overview without any effort on my part.
My advice is to try out a couple of different solutions and settle with what works best for your use case.