Treat this list as a route, not a ranking. Most people who try more than one of these cities spend one to three months in each, and the hardest part is not choosing, it is timing: Lisbon and Porto are best in spring and autumn, Chiang Mai from November to February, Cape Town from December to March. Line the cities up by season and the whole year starts to plan itself.
Flights between bases are where money leaks. Book one-way tickets four to eight weeks ahead and search a few days either side, since midweek fares are often far cheaper. Tools like Kiwi.com combine airlines that don't normally sell together, which helps on routes like Lisbon to Cape Town. In Europe, a delay of three hours or more can mean compensation of up to €600; AirHelp files the claim if you don't want to.
Before each move, sort the three things that are hard to fix on arrival: a bed for the first week with free cancellation, mobile data that works at the airport (an eSIM from Airalo installs in a couple of minutes), and a way into town at night. After a long-haul flight a fixed-price transfer is worth the small premium.