There’s no one best. Different things will work for different people. Sometimes it’s a matter of budget, sometimes it’s a matter of features.
If you want the “good enough” answer for a budget switch the older GE zwave line are very popular. They will work fine out of the box with SmartThings and are officially supported. These will have a model number that starts with “12.”
If you get the newest version of the GE switches then they have some additional features, but they are not yet officially supported. They will still work, but to use the new features you have to use community code and you might not be interested in that. These will have a model number that starts with “14.”
I personally prefer a better engineered switch, so I use Lutron Caseta switches in my own home, but those would require the additional SmartBridge device and you said you didn’t want one with an additional device.
If you want to read more about what the different features are and why people might choose one over the other, you can look at the device class features FAQ. The light switch discussion starts around post 40. (This is a clickable link.)
But if you just want a simple choice in a budget switch and you don’t want to read one more word about comparisons, get a GE switch that starts with a “12.” It’s not the best switch on the market right now, it’s not even the best switch in its price class, but it’s a “good enough” switch for most people for most purposes. 