What is the best DNS record to make www resolve to non-www - A, AAAA or CNAME?

Create an A record mywebsite.com:A=[IP address], then create a CNAME record www.mywebsite.com:CNAME=mywebsite.com. Then later, if you change IP address for mywebsite.com, lookups for www.mywebsite.com also resolve to the newer IP address.

Instead of the CNAME, you can create www.mywebsite.com:A=[IP address], and it'll have the same effect. There's not really a lot of reason to choose one over the other.

You can probably ignore AAAA records, and you might not even have an IPv6 address anyway. But if you want/need to, you can create mywebsite.com:AAAA=[IPv6 address]. Then the CNAME record at www.mywebsite.com will then resolve to both A and AAAA records.