Welcome! 
There’s no one answer Because people have different requirements, preferences, and budgets. So I’m sure you’ll get a lot of responses! The more detailed you can be about your own needs, the more helpful the responses will be.
AESTHETIC PREFERENCES
The more detailed you can be about your own needs, the more helpful the responses will be. For example, some people absolutely hate the look of the clunky box sensors and will pay extra, sometimes double the price, for a sensor which can be recessed inside a wall or for one of the very thin strip sensors that can go on the door frame instead of on the door.
Or they may want a sensor of a particular color, or with a particular LED color, or one that can be painted.
Vs
RESPONSIVENESS/SIZE
A sensor to alert you if a window was left open because of rain or closet door is left open because that’s where you keep the cleaning supplies probably doesn’t need to be as responsive as one that’s going to turn on the lights when someone walks into the room. And less responsive usually means less expensive. 
A Household with small children may prefer one of the really big motion sensors, maybe 4 inches or so, over one of the tiny ones because of choking hazards. 
RANGE AND PROTOCOL
And then there’s the question of the size of the house. Because they get better battery life, most inexpensive residential home automation sensors use either zigbee or Z wave. But one of the ways those keep the battery usage low is by keeping the transmission power low. They use “Mesh networks“ Where devices pass around messages for other devices until they eventually reach the hub. This allows each individual device to use less power.
The trick is that battery powered devices are not “repeaters“ – – that is, they don’t pass along messages for other devices because it would take too much battery power. Only mains powered devices act as repeaters. And only for their own protocol: zwave repeat only for Z wave, zigbee repeats only for zigbee.
That means you have to have a mains powered device (Light switch, plug in pocketsocket, outlets, light bulb, etc) every 50 to 75 feet or so throughout the house so that the battery powered sensors that are far away from the hub will still be able to get the messages to it.
And that means that you might want some of your sensors to use the same protocol so that they can use the same repeaters. Otherwise those very cheap sensors might end up costing you a lot of money because you it had to add two $30 repeaters to cover your space.
REGION
We also need to ask what country you are in, because the device selection does vary. 
And we should mention that most sensors will require that you also get a smartthings hub. The hub model doesn’t really matter as long as it’s at least the V2, but the original V1 model has some limitations in which sensors it can work with.
But if you do want to stay with a “hub optional“ configuration there are a few Wi-Fi sensors available, but again we need to know what country you live in.
NEXT STEPS
I know that’s a lot to start with. 
And I’m sure you’re going to get a lot of responses with a lot of specific model suggestions. for some people, this is the most fun part of automation: hearing about what everyone else is done and doing the research. But then other people just hate that part, so if you just want a quick list of “lowest common denominator“ pretty good model numbers, let us know that, too. But we do still need to know what country. And if you will be using a hub.
Meanwhile, I suggest you do the following:
- read the wireless range and repeaters FAQ. Start with post 11 in that thread, read it, and then go up to the top and read the whole thing. You can just skim it, you don’t have to know all the details, but it will give you a foundation for some of the later conversations.
A Guide to Wireless Range & Repeaters
- take a look at the device class features FAQ. Although the sensors post there was written in 2015, it still applies. Nobody’s come up with a brand new kind of sensor in that time. And most of the specific sensor models used in the examples still exist although they may have been updated for new network features. So again, it’s good foundation information.
Bulbs, switches and sensors, oh my....what to buy (device class features FAQ)
TL;DR
And if you absolutely hate long posts like this, or you have cognitive issues and can’t cope with long posts like this, that’s OK. That leaves one of three options.
A) tell us what country you are in and if you are going to use a hub or not, and tell us if all you want are specific model recommendations, not a lot of explanations.
B) or don’t get a smartthings based system. Because one of its main strengths is its versatility and ability to work with many kinds of third-party Devices. But that also means A lot of research and a lot of posts like this to figure out just what will work together for a specific use case.
Instead, you can get one of the single brand systems like Aqara with an Aqara hub or Blue by ADT or one of the Systems with only a few device choices like Apple HomeKit. Those systems work fine, they have pretty easy set up, you’ll probably never have to even visit a discussion forum, and the prices are reasonable. It’s just instead of having several dozen sensor options for a motion sensor you might have two. Or one.
C) and if even that seems like too much work, you can just call Xfinity home Or Vivint and get a system that’s professionally installed. But of course you will be paying extra to get that service.
All of these are good choices, it’s just about what will work best for you.
And welcome again!
