X-Rite i1Display Pro
Most highly regarded prosumer display calibrator.
This is a historic low price by $10USD and $20-30USD below typical sale price. B&H (who are extremely dodgy) once had it cheaper by $10AUD. Here, Amazon is the seller. So buy buy buy!
IF you signed up but didn't get your $10 mastercard credit, use online chat and get a $10 voucher applied to your account, like I did.
Camelcamelcamel:
http://camelcamelcamel.com/X-Rite-i1Display-Pro-Display-Cali…
Now the only question is what will I do with this when I upgrade my screen to a curved one? :/
FYI there are cheaper options available. Generally the hardware is the same if it's in the same series and by the same manufacturer. Hardware varies between manufacturers.
For example, there are plenty of people using colormunki with ArgyllCMS https://displaycal.net/ - but keep in mind the i1Display Pro is only $34USD more.
Also, the Spyder5's - all the Spyder 5's have the same hardware (but different to X-rite's) however the manufacturer software is gimped on the cheaper versions. You'll need at least the pro unless using ArgyllCMS.
Spyder5 Pro's can be a lot cheaper (as low as $134AUD recently) but be aware that they are considered the lesser sibling compared to X-Rite.