Travel battery pack

If you mean a power source that plugs in the wall, any Apple or other reputable brand will be fine; all of them are universal, and will work with any line current in use in the world.

If you mean as a backup power source to carry with you, again, any reputable brand. I have several; I go for 10,000 MaH or higher, which can recharge an iPhone 2 to 3 times. Ones that plug in to the phone are more efficient than wireless because there is greater energy loss in wireless chargers than there is in wired. Anker, Griffin, and other brands all are good. Avoid “no name” devices. I use a Zendure, recommended by Wirecutter; it’s 10,000 MaH and can fast charge an iPhone with it’s 20 W USB-C output: