Any good advice on which monitoring devices are best?

As with many "senior" products and services, the vendors have figured out a way to milk as much money as they can out of seniors (no apologies for saying this very bluntly). The technology for tracking a senior in their home ( or a roughly 800 ft radius) is no more complicated than that of a family dog's perimeter fence system. Yet the various senior "alert" systems have managed to advertise their systems, in conjunction with an expensive (and redundant) call center, on national TV "HELP -- I've fallen and can't get up!" and make tidy profit by charging a monthly fee around $35-55 /month. When signing for their systems, a purchaser is encouraged towards a yearly Contract; the equipment remains company Property and must be returned or paid for (at contracts end or death of wearer). The monthly fee is redundant specifically because it duplicates the services of 911 call centers which are free everywhere. Moreover, the call center merely operates as a concierge service which may delay connections with 911 personnel, instead of the assumed benefit, a more rapid response by 911.
Instead of a monthly subscription service, which costs about $550 per year (and leaves yet one more item for an Executor to resolve), I feel a one-time purchase of an "alert" pendant is a much better value, and has the intrinsically better value of immediate connection to your designated caregiver or 911, whichever you decide to program as your first call....and the devices can call a list of phone numbers that you can enter into it, before it calls 911. This means instead of talking to someone at a Call Center (that they've never met) they will talk with their daughter, or neighbor, or regular caregiver....and if those people can't be reached it goes right to local 911 call center. This represents a true cost savings, and more personalized service--who would you rather speak with in an emergency, someone you know, or maybe the fire station you've walked by for 30 years, or some Call Center 800 miles away?
The choices are out there. You can get a monthly contract and never be done paying (and still pay after you die, possibly), or you can get a once-and-done equipment purchase (roughly $225 ) and never have to pay a dime more, unless you decide to.
My parents spent a few grand on the monthly service and now have their own equipment, they only wish they had done it sooner. The choice is yours and your money is your own. No, I do not work for any of the "alert" companies. I am just a caregiver who needs to be careful for my parents, I hate to see them spending more money than they need to. Monthly service fees is just one are where seniors get a bad deal, in my opinion. Buy your own alert system and save a few tbousand dollars! NOTE: the fall alert technology, which my folks used to have, is not precise, they had multiple false-positives, and also several Real Falls that were not detected....your experience may differ of course....but my folks decided to forgoe the costly bother of false positives plus concerns of un-detected Real Falls.

Thank you , Everyone. Thanks for the ideas. Still checking things out. And, I will check out the above answers. So, anymore ideas are very welcomed.

We have a commercial system that is monitored and within minutes will notify the police if we can't be reached and they respond quickly in our area. We also have medical alert buttons but tend not to wear them. I keep mine on the hand bar beside the bath. They are too easy to set off accidentally. This costs about $50 a month.
I plan to buy a phone from consumer cellular for $60 which has a panic button and GPS location. I rarely use my phone and the plan is only $20 a month. i have no wish for anything more complicated. I can still read a map!
You really need to assess what you want to achieve and the capabilities of those who will depend on them. I know some famous and rich people have a safe room in their house where they go if there is an alarm.

A relative has installed Iris cameras and thermostats in his home. Available from Lowes or Home Depot, one or the other. It is tied into his iPhone and he can monitor activity at his home while he's away. There are many options available with the system and he was able to install them himself.

We use a Summer Baby Touch 02000Z digital color video monitor within our house. I've no complaints with it at all, but I cannot use the receiver outdoors. It will work with up to 4 cameras.

It really depends on what exactly you are looking for. We had a Lifeline medical alert system for years while my mom was still living home alone. She wore a panic button and could summon help if she need it for any reason. It wasn't cheap but provided peace of mind for all of us. And as she is visually impaired I set her meds up in a dispenser for her, my uncle had "compliance packs" of his prescriptions delivered weekly from the pharmacy (for a fee, of course). I think once you move into the realm of door alarms and surveillance systems your elder is probably not safe to be home alone.