We do not have a particular recommended printer. PrintNode is designed to be compatible with any printer at all, because it is ultimately itself just a transport, getting a print job across the Internet, past firewalls, to your existing printer setup. It is unfortunately not economic for us to invest in testing different printers, given the pricing expectations of the WooCommerce extensions market.
If you already possess your printer, then note that (at the time of writing this FAQ at least), PrintNode offer 1 month of free service. You can simply sign up for a free account, and test it directly.
So, any printer at all that you can connect to your computer and print from is suitable for use with this plugin. We are aware of customers successfully using a wide range of makes and models of printers, including different thermal printers.
We first tested and developed the first version of this plugin using (among other things) a Star TSP-100 thermal printer in 2013, and are aware of customers happily using this printer still today several years on, but this does not constitute a formal recommendation.