I use Simplisafe. It was affordable and the monthly monitoring fee is one of the cheapest. BUT.... do I like it?

Well, yes.... but not having any knowledge of other systems I would 'guess' it's not one of the best. I could be wrong, maybe it is. It's certainly one of the cheapest.
Currently have all windows, doors, with sensors. Also some motion detectors and glass break detectors. Plus 3 outdoor cams... one is a video doorbell. The video doorbell was a challenge to install.
My home is old (1993) and the original doorbell transformer didn't have enough power to run it. I was lucky to find a package deal... new doorbell and a more powerful transformer. Installing was a lot easier than I thought.... although next time I'll shut the power down before working on it.

No, didn't shock myself but it wasn't the wisest way to change the transformer.
Patting myself on the back... I was brilliant in deducing that the reason the video doorbell wouldn't work was due to not enough voltage... there was NOTHING in the manual that mentioned that would/could happen with older doorbells.
One complaint... the system has sensitivity settings BUT... not the best. Too many notifications on some things. With indoor cats I cannot use the motion detectors, (unless the cats are locked up), because they will set it off every time EVEN THOUGH I have the sensitivity set to ignore pets.
I would recommend Simplisafe BUT... I would NOT give it a top rating. - JamesJM