What is the best printer for Ubuntu?

In general Linux does not require specific drivers for printing. CUPS will handle this for you. The generel rule applies here: stick with compatible hardware and you will have a hassle free usage.

Regarding specifically printers:

Have a look at the Ubuntu wiki page on printers. Notable excerpt:

Manufacturer-Specific Installation

Many of these printer manufacturers require non-free drivers or plug-ins. HP has the largest selection of free software compatible printers. Some of the higher end laser machines from other manufacturers are at least partially free software compatible.

And the wiki has a link to a site that sells Linux compatible hardware

• https://www.thinkpenguin.com/catalog/printer-all-one

I do not think you will go wrong when you choose any of the other printers listed on these 2 sites or even if you just buy a random HP printer.

hp is the best Linux Imaging and Printing

HPLIP is an HP developed solution for printing, scanning, and faxing with HP inkjet and laser based printers in Linux.

The HPLIP project provides printing support for over 2,080 printer models--including; Deskjet, Officejet, Photosmart, PSC (Print, Scan, Copy), Business Inkjet, LaserJet, Edgeline MFP, and LaserJet MFP.

HP Linux Imaging and Printing

I'm using a Brother HL-2270DL Laser printer ($69) with no issues. Just plug it in and go. It is also wireless , but I didn't try it in that mode.