LOL!
OK, OK I was going to leave this for other people to answer as I’m not feeling very well today, but here I am.
First the long answer. 
There’s no one best because different people have different needs and preferences – – and budgets!
You might first take a look at the device class features FAQ, because it will explain what some of the different features are that a model might or might not have, and that can help you decide just what you might be looking for.
Short answer 1: Lowes
Now the short answer. If you want relatively inexpensive, reliable, officially certified to the same protocols that SmartThings uses, and you’re OK with zigbee sensors, the community consensus seems to be for the Lowe’s Iris sensors, particularly if you can get them on sale or if you can use an in-store coupon.
Be careful because Lowes sell several different models, and not all of them work with SmartThings. So read the reviews to make sure it’s the right model. And write down the model number before you go to the store if you going to buy in store, because the clerks won’t have any idea of the difference between the different models.
https://www.lowes.com/pd/Iris-Iris-1-Sensor-Indoor-Door-And-Window-Sensor/999925302
Short answer 2: some zwave options
If you want Zwave sensors, Econolink seem to be the fastest inexpensive zwave model ( still slower than the zigbee ones, though) , but the gocontrol/Linear/2gig are often on sale at Home Depot in a box set of 2 contact sensors and one motion sensor, and can end up being a little cheaper. All of these are reliable and officially certified, it’s just that in general Z wave sensors tend to be slightly less quick to respond than zigbee sensors. Also the battery life won’t be quite as good as with the zigbee sensors, and the sensors themselves are usually bigger.
Short answer 3: cheap but flaky
If you want super super cheap but occasionally flaky sensors that are not certified, you could take a chance and go with the Xiaomi brand of zigbee sensors from China. They seem to be pretty well-built, but they are intended to work with their own gateway not anybody else’s, and, like I said, can be flaky.
But if you want to consider the Xiaomi, make sure you read through the following thread first. Yes, these are super super cheap, often $10/sensor, but you will see that people also have a lot of different kinds of problems with them. So know what you’re getting into. Also you probably have to buy them from gearbest, which is a Chinese importer, so they can take a long time to arrive and the returns policy is not as good as Home Depot or Amazon or Lowe’s.