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A glib answer is arduino MKR or Raspberry Pi.
For a more appropriate answer you need to supply information about your level of expertise and what you want to connect to the internet.
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A good all around processor is the Pi 4 because it is well support and very good value for the money. But any given details can change that answer of course...
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"Best" has a lot of wiggle room in it.
Best price would be ESP-01. It's hard to program and requires special 3.3v power input.
Best price for an easy-to-program and easy-to-power board is likely the Wemos D1 Mini.
The D1 Mini has limited I/O, which is best if you're using it just for a few sensors.
But if you need the best price for something with much more I/O than you might prefer the Lolin v3. That might be best for more complex robotics to have more sensors and more controls.
Brand name versions of the above are available from Adafruit, in the Huzzah feather and ESP8266 breakout, and also in the Arduino MKR line.
The Arduino MKR Wifi boards are great too, as Douglas mentioned above, but they are another step up in price, but might be easier for a beginner to connect to the cloud, using Arduino's cloud services.
As Doug and Clem both said, the Raspberry Pi 4 is probably the most versatile, yet still quite affordable. It's probably the best one to offer all sorts of different IoT solutions and learning experiences.
But I wouldn't consider it "best" for monitoring a single temperature sensor, for example.
So I suggest you choose your own definition of "best" and see what fits