Much depends on your weather. I live near San Francisco, and there are a number of devices I can use in a sheltered space outdoors. Someone who lived in Minneapolis would have fewer.
The easiest button to connect to smartthings right now is probably the iris button. It’s one small square button that will connect directly to the hub and costs I think around $25, but you can usually get a coupon to bring the cost down 10 or 20%.
The Aeon panic button is a one zwave button key fob which can also connect directly to smartthings. That one is officially supported which is always a plus. But it tends to cost just under $50. It also looks a little more like a doorbell, maybe because of the contrast.
There are a number of other options including one of the battery operated light switches.
Personally, I like the flic buttons, but these are Bluetooth so you have to have a Bluetooth device set up as a receiver. That works at my house because we have a couple of tablets set up permanently as home automation controllers. They also cost more than the Iris, $30 - $35. But one of the reasons I like the flic is because my service dog can use it. That’s not an issue for many other people. 
So is always different things work for different people.