.co.uk is the cheapest, which makes the most sense at the moment (only £8 / yr) but .com is most universal (£12 / yr). I wanted a .io because Input/Output makes so much sense online but it's £40 / yr. Depends how much I use it all, I guess. 

 @024f08ef there are cheaper domains out there:
https://shkspr.mobi/blog/2022/09/whats-the-cheapest-domain-you-can-register-for-10-years/
But it does vary depending on the whims of the registrars.